Comment by tart-lemonade

Given enough time, you'll end up with a lot of legitimate users who follow a huge number of accounts but rarely interact with more than a handful, similar to how many long-time YouTubers have a very high subscriber:viewer ratio (that is, they have way more subscribers than you would expect given their average view count), and there's nothing inherently suspicious about it. People lose access to their accounts, make new accounts, die, get bored, or otherwise stop watching the content but never bother unsubscribing because the algorithm recognized this and stopped recommending the channel's uploads to them.

Bluesky doesn't have this problem yet because it's so young, so the outsized follow counts are mostly going to be from doomscrollers and outright malicious users, but even if it was exclusively malicious users, there is no perfect algorithm to identify them, much less do so before they start causing performance problems. Under those constraints, it makes sense to limit the potential blast radius and keep the site more usable for everyone.