The reason people still play on PS4 is incredibly simple, there are barely any exclusive PS5 titles.
The PS+ catalog often contains not only PS4 titles, but PS4 titles from as old as 9 years ago. Why should people bother? (E.g Elder Scrolls Skyrim, The Witcher 3, Kingdom Hearts)
If you really want to see evidence... go back to the PS5 Pro announcement, they used The Last of Us Part II as one of their big improvements. A game released PS4, then "remastered" on PS5. Rumours are that The Last of Us original is also receiving this supposed upgrade. So a game released on PS3, remastered on PS4, then re-remastered on PS5...
So, why use PS4? Because, much like Sega with the Mega Drive/Genesis, Sony refuse to move on from it. There is no incentive for the audience because the software remains active.
But, everyone knows it's faster and better, why not upgrade? That's what you all say, right? Well, how many people upgrade their kettle, toaster or television just because a newer faster one is at market? I don't know any.
The kettle will bubble, shake and spit, but still boil water. The toaster will glow, be seven inches deep with crumbs, but still toast bread. The TV will have visible light spots, scan lines or clipping sound, but it'll still display inages.
The PS4 will be louder than an aircraft, slower than a security checkpoint in Venezuela and low res as a ten year old laptop running Cyberpunk. But, it still playa new software.
That's why