I mean, fighters do lean into kicks. Just generally not with such extreme slips, but that's an animation issue rather than a principles one. Like, you lean into kicks when you catch them a lot of the time. It's a pretty simple and quick guard adjustment, like if you want to get a feel for why you would do it, put up a loose boxing guard with your knuckles at your cheekbones then lean over one hip a bit and it will drop your elbow below your ribs and tight to your body, shrinking the target for a clean body kick while also slightly moving your body away from the kick.
As for ducking/leaning leaving you open to things, correct, it does, but so does trying to catch kicks because you aren't blocking your head, I imagine that you're open to a similar number of strikes, if not more, as you are from leaning or ducking, the only thing that's different is vulnerability which in my opinion is an acceptable trade off for removing the timing requirement and allowing you to land a combination rather than 1 strike.
And honestly, I don't think using real life as an argument holds any water if it's not a consistent position. Like, you're fine with an unrealistic mechanic in fighters not being able to move or block with a held leg, but you don't want to encourage the unrealistic mechanic of ducking/leaning into kicks? Which to be clear, I do admit isn't a realistic counter to kicks, it's more of a panic last line of defence type thing to take a little bit off, like rolling with punches is.