Because immunity varies by disease.
Chicken pox? Pretty much one and done. COVID? Falls off rapidly after 3 months, whether you catch it or get the vaccine
Plus, every mutation is a dice roll on how much existing immunity will apply. It could be exactly the same as the last strain, or the old immunity might not help at all
I think it's a combination of less presence/engagement in niche content, sorting methods not quite being there, and not enough discovery without going out of band to find things
One thing I really miss is the science groups, askscience always had great debate that I haven't yet found here