Because women's liberation. It's probably not a good idea to call anybody a poetess, a sculptress, an authoress, or an aviatrice unless you know they find it acceptable. On the other hand, waitress, actress, and countess are all (so far) perfectly fine.

I don't have access to sufficient resources right now to cite sources, but from my own experience in literary circles, "poetess" has long fallen out of favor. The word "poet" is preferred because it is not even etymologically masculine. The word "poetess" is seen as a distasteful hypercorrection at best and actual sexual at worst.

@PeterShor - yes, waitress, actress etc, but what’s the problem with poetess? (Btw Spanish, French, German, Italian etc all use female version of poet). Does the word “sound” somewhat disrespectful?

@RMac - “The word "poetess" is seen as a distasteful hypercorrection at best and actual sexual at worst.” ok, but why.

@PeterShor Although simply calling actors “actor” regardless of their gender is currently in favor.

As someone with a PhD in English literature, I haven't heard anyone alive call a female poet "poetess," except in reference to someone dead who had.

@TaliesinMerlin - and there must be a reason for that.

The reason this kind of language has fallen out of favor is that today, gendered language is seen by many English speakers as pointless at best. There is a good reason to describe someone as a person who writes poems (a poet), but no good reason to describe someone as a woman who writes poems (a poetess). I.e. there's no important difference between a person who writes poems and a woman who writes poems, so there shouldn't be two words.

@PeterShor, although in many places "waitress" is still acceptable, some eating establishments have converted to the neutral "server". And on a personal note, I find the Hollywood orthodoxy's insistence on "actor" referring to both men and women to be offensive. If I am a lead in a film, I want to know my love interest is going to be played by an actress, not just any actor. When it comes to filmmaking, gender does matter.

@user66974: French, Spanish, German, and Italian are all gendered languages. English is not. If you call somebody a poète in French, it's a masculine noun so you have to refer to them as he after that. To avoid this, they have the word poétesse, which is the feminine version of this.

@PeterShor - well, you still use he and she, his and her, hopefully.

@PeterShor - I think we’ve lost the thread. Probably I shouldn’t have asked this question.

I'd question the premise that "poetess" was ever preferred. This Ngram shows poetess at a virtual flatline compared to poet; it was only twice as common in the 19th century. And even in the 19th century it was common to refer to female poets as poet: "Elizabeth Barrett Browning, one of the most gifted of female poets"

It is flawed to think that poetess must be used over poet because the feminized term is available. Even when poetess was more common, female poets were often called poets.

@TaliesinMerlin - who said that poetess “must be used over poet”?

It's an assumption of the question. "Why do they call her poet?" Because she's a poet, and that term has always been acceptable to refer to her. The question would be improved by introducing the notion of frequency - "Why is poetess less common today?" for instance. That would also avoid some of the possible axe-grinding implied by some of the comments. (For instance, "poet" is not the male version. English does not have a "male" version of poet. It is the general version.)

@TaliesinMerlin - no sorry, there is no assumption on what term should be used, but simply a question on why a male version is preferred over the female one. (An issue that appears to be disturbing I guess).

The actor/actress question asks whether it is OK to use the traditionally masculine term for a woman (and receives the correct answer that it is). This question asks whether it is still OK to use the traditionally feminine term for a woman, and if not, why not. These are very different questions.

The real duplicate is Should I use authoress for a feminine author, which answers this question. Unfortunately, if we vote to close it right now, it will get closed as a duplicate of the wrong question (which already has three votes to close). So unless some moderator can close it for reasons of the right duplicate, maybe we should just leave it open until those close votes go away.

@PeterShor - actually the accepted answer says “ The ‘male’ form is increasingly being used as the ‘neutral’ form, where the gender of the person concerned is simply unspecified.” which does not appear what you and other users have been saying so far.

@user66974: It says "In the late 20th century, as the role of women in society changed, some of these feminine forms became problematic and are now regarded as old-fashioned, sexist, and patronizing (e.g., poetess, authoress, editress)." I think this is exactly what we're telling you.