Why do most newer phones other than pixels have little to no support for custom ROMs ?

I know that most customizability features that were once exclusive to custom roms are available for use right out of the box these days.

In the case of manufacturers like Samsung, I think,there are still no official builds of lineageOS for the newer phones after the galaxy s10 series.

I'm aware that GSI roms are available. My experience with GSI's have been kinda bad. Most of the time they lack a lot of features which makes the phone not viable anymore. Then there are also the random UI bugs, which frustrate the hell out of the user.

I miss the old days when there were lots of custom roms, even for budget devices. I used to flash them when my phones were out of warranty. I could use my phone however I saw fit.

Is there no way to bring back these good times ? Or will the whole custom ROM community just shrink to the pixels and a select few devices ?