This is why it’s increasingly more common for streamers to not use alerts for followers as they realise it can be a vector for abuse with little way to mitigate, that follower count isn’t a metric of any real importance, and adds nothing to the value of the channel itself.
While there have been attempts to mitigate follow bots, but they all have issues. For example, blocking a user prevents a follow, but that doesn’t help after the fact. Bot lists exist, but new accounts are created far faster than any of these lists update, and they contain false positives. There are also bots that advertise they can stop follow bots, but again these just try to block bots after they mass follow which is too late and there are some bots specifically aimed at this which are known to have false positives which they don’t correct.
Because of this, attempting to prevent followbots takes time, effort, with a high error rate that will negatively impact legitimate users. Or, the streamer could just not use follower alerts and not need to care about the bots at all as like I’ve previously said the vast majority of follow bots do not disrupt a stream in any way other than following, which doesn’t do anything if you don’t have follower alerts.
Finally, streamers trying to remove followers has lead to them seeking out tools to block users which removes the follow, but there is a significant number of streamers who end up removing legitimate followers, or worse removing EVERY follower. So by going against the advice to just ignore the follow bots, many channels have negatively impacted their channel far more than the bots ever could.