With me it is simple, I now have some of the bikes that I couldn't afford when they first came on the market. Back then, it was all great technology for me and a lot of it still is today. What's more, you can get a lot of them very cheaply, which quickly turns one retro bike as a gimmick into a whole collection. Another plus point is that with an MTB from the 90s, the trails around here become challenging again for me.
On my enduro bike, everything is a bit bland and I would have to go to a bike park or the Alps every time to push the bike to its limits, which I simply don't have the time for.
Retro bikes are also easy to understand and maintain, no suspension, hydraulics or dropper posts.
I am mostly looking for rigid steel bikes but some nice titanium is also welcome if it wasn't for the huge price tag.
I am not into collecting road bikes because I don't like riding drop bars that much and only beeing able to ride on roads is also nothing for me. I find them really beautiful though.