That's hard to enforce with federation, how do you specify the limit of single user instances vs big instances like lemmy.world? You receive all of it through the same server, and you may have hours of activity backlog queued up if your server had federation issues or was offline. They'll come pouring in as fast as the remote instance is willing to send them.
If you apply the limit on specific instances, you may end up with instances where the admin runs bots that may bust the limits and it may be high or off as a result. Or the admin's just like "eh, I don't like limits".
Doing security in the wide open is hard. It's trivial to observe things like shadowbans with Lemmy.
Even with ratelimits, that also wouldn't deal with the issue with old abandoned Mastodon instances like we had with the japanese discord spam a couple weeks ago where the made accounts across the fediverse.