The answer is simple: Even a single popular subreddit has more users and content than all Lemmy instances combined.

That ‘mass migration’ a year ago made Lemmy viable as a social network, but barely affected Reddit at all in terms of numbers, and numbers are all they care about.

I’m still here on Lemmy, but to the Vast majority of Reddit users, nothing has changed (at least not enough to leave the place where all their communities are)