This is basically the same situation as the UK. I won't answer as I have no specific insight about Thailand, but if their system is similar then all entries/exits are recorded electronically regardless of entry/exit method, and the stamps are just decoration (in most cases).

Because it's difficult to do for a machine? What's the question here?

@Relaxed Why do immigration officers place an exit stamp on the passenger's passport then?

Why not? Because it's the law? Again, what's your question? Why stamps exist? Based on your question, this does not seem to have anything to do with the machines, indeed you wrote that you couldn't use the machine because someone in your situation require a stamp.

@Relaxed "Because it's the law?" idk, you tell me

If that's what you want to know then just remove the nonsense about the machines from your question. But it's not news to you that countries stamp the passports of some visitors, surely?

@Relaxed Not nonsense. That's what made me wonder why humans add exit stamps if machines don't. "But it's not news to you that countries stamp the passports of some visitors, surely" most countries don't have eGates.

You're framing this question in a completely backwards way. The questions that do make sense is (1) when and why are stamps required and (2) who is eligible to use e-gates. Many countries have eGates and usually restrict them to people who do not need stamp, have an officer apply a stamp at the exit of the eGates, or phased out stamps completely.

Machines don't stamp passports because it's too difficult (requires going through the pages, finding a blank spot, angling the stamp, etc.). Machines that do that do not exist and would be inherently costly and unreliable, with a risk of damage to the passport. The only thing that would be mildly surprising would be a country that gives the option to get a stamp or not by choosing the machines but your experience in Thailand is exactly the opposite. Again, many countries stamp passports (including some with eGates), you can ask why that is but that's completely unrelated to your observation.

@Relaxed Thailand says anybody can use the eGates on exit. So it's not a matter of "needs a stamp -> needs to see an officer". Here for some unknown reason it didn't work through the eGate (but presumably, if they didn't change policy, not because a stamp is needed), so OP saw an officer, and officer stamped, while if things had worked as intended he wouldn't have had a stamp. Is that clearer?

@jcaron It's not really clearer, simply different. It could be a reasonable question but it is not what the OP is describing. My own research suggests access to the eGates for foreign passport holders is relatively new so obviously policy has changed.

@Relaxed "simply different" what's the difference?

@FranckDernoncourt You're focusing on two red herrings: why machines do not stamp and why stamps exist, both trivial questions and completely unremarkable things. You also do not appear to know for a fact what happens when you complete the eGates process as you couldn't do it. If the question is “why stamps are optional / could apparently be avoided by going through the eGates?” (not that you were actually able to do that, mind you) or “why are officers still stamping even though stamps are not always applied [anymore]?“ then ask that.

@Relaxed isn't your suggested question "why are officers still stamping even though stamps are not always applied [anymore]?“ the same as the question I wrote "Why do immigration officers place an exit stamp on the passenger's passport while automated emigration gates don't at BKK airport?"?

No and I already explained why at length. I do not know any automated gate that places an exit stamp (and I actually work for a company making some) and I know countries that do have officers stamping passports after processing people through an automated gate (e.g. France, Netherlands). If and when automated gates are used without phasing out stamps then the stamps have to be applied manually. Why automated gates do not stamp passports is a trivial question and reminds me of all these other questions you seem to dump on the site with zero research or thinking.

For the "Needs details or clarity" VTCs, what else shall I clarify?

“Because we’ve always done it that way” is the default answer to these questions. Trying to find logic behind a bureaucracy is futile.

@FranckDernoncourt I submitted an edit to the question which (I think) clarifies what your question actually is and what it is not. Feel free to edit or revert if I did not correctly convey your intentions. Though I think JonathanReez is correct: it's just a matter of "we changed the rule for the eGates but we just haven't changed the rule for the agents (yet?)".

@jcaron Thanks very much, it correctly conveys my intentions. He's likely correct indeed, but I prefer when my knowledge is more certain than likely. E.g., maybe humans only put a manual stamp and don't add that info in their computer system.