main reason: anti user freedom, and locking you in to their system, it’s extremely hard to wipe out your phone in order to sell it if you have a Samsung account linked to your phone, and they make it hard to flash a custom ROM, imagine buying a phone with your own money and you still need the manufacturer consent to do what you want with it…
confusing and slow UI
Ads everywhere on the UI
bloated with games and useless apps
they don’t take security seriously at all ( slow updates )
short update period
they lie in their marketing by giving big numbers ( battery capacity and camera quality for example )
Huawei, Xiaomi and Samsung phones
And last but not least, they kill your apps