Supply of retro bikes (or at least the ones I want in the big sizes I need) has got a lot smaller in recent years. There used to be a time, 5 years ago or so, when you'd see the same handful of top flight frames being cycled between different RB members and there was always the illusion that maybe, maybe next year I'll but that frame or whatever when it's current owner decides to move it on. It feels like the market slowed down, prices fell, and some of us (including me) are scared to sell because the chances are now I'll never find another one in my size again if I let my Rourke / M Trax / Dynatech go.
I also reckon lockdown has, as others have suggested, led to more people at home on the internet dreaming about bikes and deciding to buy what they hankered after or rode in the 90s. Plus travel on public transport is being discouraged in many places, with cycling seen as the way forward, so a lot of people are buying themselves a bike to commute on, and a rigid retro can be a good option for that. SO, an influx of money combined with restricted supply is probably what is pushing prices up.
These things go in cycles, so hopefully the high prices will stimulate people to check the back of their garage for their old bikes to sell, boosting supply and dropping prices.