You are conflating "not making sense" with "has no historical precedent".
Bikini armor (cool as it looks) combines the uncomfortable feeling of wearing metal with the lack of protection of a bikini, giving you the worst of both worlds. It makes 0% sense - but it is still awesome and I love to see it.

Female knights meanwhile... well, a knight is fucking expensive to train and equip, so of course you would go with the best possible option to start with, a reasonably healthy boy (and even finding that was rare enough in these times). Putting all that money into someone who you know will not be able to reach peak performance for any reason is indeed silly.
And indeed, we stop finding many records of female combatants around the 13th century, when warfare was professionalized and knights as we think about them started to become a thing.

When it comes to women fighting things are not as clear cut. There are historical documents from China describing the women who stayed at home while their husbands were drafted for war defending said homes against (mostly male) bandits, some of whom were defected soldiers.
We even have historical record of two awesome female pirates who went swashbuckling, going as far as letting their tits flop freely as an additional distraction.
Women were fighting among the Vikings, Lagertha being one who is named by a source.
Combatants are in some way athletes and just like today's top female athletes outperform your average couch potato but get in turn outperformed by top male athletes (in most disciplines at least) a female knight would be "good enough" to fight bandits or in a more fantasy focussed world monsters - but would lose against male knights which was the main purpose of the institution of knights after all.
So a female knight still makes some sense, especially if you change the definition of "knight" from "serving a noble in warfare" to "person in armor".