Why do fans always freak out when Idols start solo activities thinking it's the end of the group?

Neither do I. Thankfully it hasn't been like that for groups I stan like BLACKPINK and Twice, and Stray Kids (kind of?). Of course there are a few doomsayers, but most of the people I see are excited for more music. That being said, that mentality of freaking out and thinking the group is on the verge of disbandment because of it is hilarious.

Not saying I believe it but I think I understand the reason behind it.

I think it has to due with the idea that if resources and time is allocated for solo work, that its taken away from group then this ties in with the idea that less group activities = declining group = group is over soon.

Solo work doesn't have to mean the group is put on the back burner, but it usually does, hence the association. As for the group being "over," you don't tend to see idols go solo right out of the gate. It's usually when their group has been around for a while and is potentially starting so slow down their activities anyway, which is what tends to feed those fears.

Besides sm groups I think that is a legit fear

Quote from Ramasson

Besides sm groups I think that is a legit fear

I think it's a justified fear, SM balances it very well, but not every company can. So it's very common for a member to become very popular and surpass the group's popularity. Then people only look at that person's solo, not the group.