This is because a lot of people didn't unpack their inherently cisheteronormative thoughts on topping and bottoming. In their mind top = man = masculine = dominating (but not in bdsm sense) and bottom = woman = feminine = submitting (again not in bdsm sense more in degrading way) Which is why you see people fighting over who gets "feminized" in slash ships and why it is wrong to feminize them.
And here is where comes the "I see them switch because they're equal" conversation which is almost saying the quiet part out loud. That they think one of them will be degraded in the fixed top/bottom dynamic so the correct way is making them vers/switch to avoid degradation/feminization.
The truth is everyone should write whatever they want. But because now everyone is super into adding moral qualifiers on why they write/don't write certain things queer people have to sit through this top/bottom discourses and hear how wrong fixed top bottom preference is and how "heteronormative" it is. Often from other queer people. Funny isn't it (it's not, it's exhausting).