Migrate is a strong word, more like you can copy your settings, it's certainly not intrinsic to the standard either. Remember, your username includes your instance after all. I am not deeply versed but one of the better platforms was Mastodon and it was still limited to moving your settings and followers to your new account and setting the old one to forward. Posts weren't reattributed or anything though.

The strength of the fediverse is how the data is duplicated and how it's the sum of many parts, but maintaining an "identity" on the fediverse is still wobbly, some like that though, it leans into the privacy/anonymity side of things.