OMG

it's a long thing to talk about.

But we can say the reason that tends not to happen is because yaoi is written by women to women, the uke it's supposed to be the woman and the seme the man. So having that mindset on mind, you will find the answer. If you start the story with the heteromartive mindset from the start, you can't just easily say 'oh they can switch'. (I guess, yes, you can, but they just simply don't do it).

Now, saying that, since it's written to women, it is important that the uke remains the uke. So that those young female characters can relate to the uke, and wish for them to have that imaginary perfect "seme" (just like disney's princess movies but... instead of princess just ukes lol)

It's not that ALL yaoi are like this, of course there are always exceptions. There are some versatile characters and reverse coupples.

But that's the conclusion I got to after reading some post about this, and my own thoughts.

I have to add tho, these line of thinking does not only apply to straight men or women. There are gay guys (I'm a gay man myself) that are only 'top' because being a bottom is girly. Like... man come on, but well, that's a hell a lot of other problem, but I think is worth mentioning it.

Damn I fucked up, i wanted to say on the second pharagraph "So that those young female readers, can relate to the uke characters, so they can wish for the perfect seme (just like disneys...)

And for some reason, I'm not even surprised... :v

life is tough