Every extra kg flown costs extra fuel, so if you want to take more kg they charge you more. Maybe you can try to negotiate to take an extra kg if you take a dump before heading onto the flight ;)

You might like to ask this on Aviation.SE as well, where they would know more about any technical reasons for this rule. I have a hunch that algorithms for estimating weight and balance might be a factor, but I don't know enough to say.

Given the variability of policies across airlines that fly the same aircraft, it's unlikely that there is any technical reason. Most likely it's money: they want more people to check bags and pay a hefty fee for this.

You only know how full the plane will be when the gate is closed (no shows, people who missed their other flight and switching/purchasing this one, staff flying on short notice).

On some airlines, I can imagine they might let you carry overweight bags, but not officially. As in: they will say 7kg, you bring it up to the gate, they announce something to the effect of "please check in overweight bags at the gate, we will be weighing them" (as usual), you think "damn, not this time" and get it weighed, they tell you "oh it's okay, we're empty, just bring it on board", and of course they never weigh bags at the gate even when they say they will.

Probably to avoid disputes.

I always thought it was to accomodate a 2-person lift for heavier bags, give the baggage handling folks a heads up that a bag could be too heavy for one person.

So you won't start thinking about murdering co-passengers to save on overweight fee.

@user253751 re : ... and of course they never weigh bags at the gate even when they say they will ..." -> I have seen bags weighedin no-man-land after entry to international entry gate and before security so that people have NO option but to abandon items. This could hve been done at the entry point so an option to return stuff to the land of the living was available. My comparison of people and airlines that did this to people from regimes some 75-80 years back had my answers threatened with deletion previously so I won't make such commemt here.

@RussellMcMahon I'm not sure which area you're talking about. I don't know of any area where you can't exi, and I don't know why you couldn't go through security anyway and then ask the airline staff to gate-check your bag. (If you exit a secured area then of course when you come back in you must go through security a second time, but that's hardly a problem)

@user253751 This is on international flights. I've seen that happen in NZ and other countries. The impression given to most (and me) is that once you have entered the areas on an international flight where non-passengers may not enter then o leave would require you going through exit security. That may not be sensible and may not be factual, but if not, many do not realise. I have seen people being compelled to discard property in such circumstances. If redress was available it was not made clear to the victims. |...

... If travelling with an allowance of one checked bag (as is always the case for me) then an additional bag checked adds $80 say to the cost (that was the fee on the trip I'm on at present (NZ - Australia- NZ) if I'd wanted an extra bag). That's regardless of weight. I have never seen or heard of anyone "gate checking" anything. It may happen within the US but I'm not awar of it on numerous international flights. A person in a secure area being allowed to leave would have to be security checked as otherwise it opens loopholes for smuggling.

@RussellMcMahon I've never heard of "exit security" either. Usually there's a one-way door and someone watching it to make sure you don't go the wrong way...

@RussellMcMahon plus my first point still applies: there are airline staff at the gate, you could get to the gate and then ask them to check your bag. (I've never gate-checked a bag, so idk how that part works)

@user253751 As I noted above "This is for international flights". And, as the OP specifically mentioned China Airlines, the implied focus seems to be more international than in-country. || "exit security" is usually called "Customs" & in some cases (eg NZ) also "Agricultural inspection". || As I noted - "gate checking" a bag with eg Virgin Australia, (who I just came home via a few hours ago from Australia) would, if they would do it, incur an additional bag charge - $80 in the specific case of VA between NZ and Australia. ...

.. As I have NEVER heard of gate-checking a bag in my travelling between 12+ countries that I have flown between at various times, it may perhaps be more a US thing. I'd be immensely surprised if eg Virgin Australia (something like an antipodean Ryan Air) would respond usefully to a request yo gate-check a bag.

@RussellMcMahon Ryanair does a lot of gate checking often against the passenger's will, or for a fee if the bag is overweight. At least they used too, it's probably rarer now with the recent changes in bag-checking arrangements, but I haven't travelled with them in a while.

@RussellMcMahon I've seen China Airlines doing weight check at the gate (even for non-obviously overweight items) in the US (SFO, boarding to TPE).

@RussellMcMahon I've always heard of gate-checking bags that are overweight, and it always says that on the sign with the size tester too. "If your bag is oversize or overweight it must be checked at the gate." (in practice this only applies to bags that are very oversize/overweight)

The real question is - why do they weigh the luggage but not the passenger?

@GrimmTheOpiner Generally for the passengers, average weights are presumed. For baggage, especially that which you check in, with what is allowed and what people may be travelling for, there can be more variance with the weight (I once travelled with cast iron pans that added a whole 10 lbs to my baggage)

Only Sam Chui gets to enjoy such benefits, he's then usually allowed to fly free of charge, or even paid to fly. Airlines contact him whenever they see there are lots of empty seats on flights that they can't fill.