For the same reason we have every fucking other problem in the world right now.
Capitalism.
You cannot align incentives towards growing the wealth of a microscopic yet astronomically lecherous group of monsters and expect society to not just crumble under the weight.
Why would companies care as much about the minor brand value decrease in the eyes of more technical users when they can make more money right now by making things shittier?
Basically, because we don't own our devices. We are allowed to use our devices by the good graces of the manufacturers that charge a premium for them.
This really needs to change. I remember the preinstalled app antitrust suit(s) in the early 00s. Those need to happen again, but likely the EU will have to as the US is entering a dark age, and the US will continue to have inferior everything to the rest of the world for the foreseeable future.
The problem is that most phones don't support ROM's like Graphene or Linage, as people installing custom roms is a relic of years past. Next time you get a phone, think about if you want this option for yourself. Also, do keep in mind that some phones have HW backdoors on them, so even installing a stock rom/graphene does not save you from privacy.
Companies want to spy on you and make money, it's no secret. Just install stock Android or your favorite fork; it quite literally takes 5 minutes with a WebUSB installer.
This is most prevalent on androids, even stock pixel. As much as I like to shit on apple, they have less bloat on your phone. Of course whatever native app you have on there, you wouldn't able to delete, whereas on android you could delete it if you really wanted to.