Didn't use to be that way. Android phones with Qualcomm SoCs used to be incredibly easy to modify with alternative versions of Android. That changed around 2014-15, can't remember why.
Now it seems only Google's own Pixel phone is the only one that's hackable enough to run LineageOS or /e/ or other de-Googled distros.
LineageOS has active support for 200 devices, and many more if you're up for some hacking, and download the ROM from the XDA forums. /e/ sells phones preinstalled with /e/OS, Pixels, Samsungs, and the Fairphone [0]. CalyxOS can also go on a bunch of phones.
Now it seems only Google's own Pixel phone is the only one that's hackable enough to run LineageOS or /e/ or other de-Googled distros.