I don't know why people insist on "future-proofing" in a market where the high-end becomes midrange in no time.
There are a few reasons.
First is that you're bottlenecked by your slowest part. I could buy a 3080 for my current machine, but there's no point on a system with an i5 from 2014 and 8gb of RAM.
Second, crypto might be coming back. Etherium keeps avoiding proof of stake. People don't want to be stuck with a 1060 for another decade.
Then there's laziness. Since a GPU is now a big ticket purchase, buying a new GPU every few years doesn't make sense. Especially since you're likely going to bottlenecked by something else like CPU or memory speed anyway. Better lay out double the price now, enjoy high end today and mid range tomorrow. At least that way you don't have to mess around swapping parts and messing with drivers.
Then there's no cheap cards worth bothering with. Tech reviewers aren't helping by testing the latest games on Ultra. Back in the day when I had a geforce 5500, I used to buy a £40 card every few years. Such cards don't really exist any more.
Recently the Radeon 6400 came out, intended as a small form factor card good for people who buy old work stations and retrofit them into cheap gaming PCs or small form factor builds. Tech media has complained about the card for not matching up to desktop cards like the 3050 or 6600.
And as
@AmpleApricots said, IGPU/APUs have got to the point where they can play most AAA games at 1080p medium comfortably if you want to go super cheap.
To power level a bit. Pre-pandemic, a family member asked for my advice on buying a pre-built PC. It was a good deal all told, but I advised against getting paying the extra for the "gaming" version with a 1030. They ignored me, and complained afterwards that this "gaming" PC wasn't very good at playing games.
Finally, I want to mention building a PC, any PC, is like buying a new car. When a new car rolls off the lot, it's value halves. A PC will be "obsolete" in 6 months. There's always some new GPU or CPU or storage improvement.