Except the Android of today has not much in common with the Android from ten years ago. Android might be open sourced but is by no means like the linux that you'd run on your PC and is being increasingly built to wall you into Google's services while Samsung and the rest try to remove Google as much as possible from the equation and sway you to their services. Basically you're the cashcow they try to heave around. What a mess to be in.

Oh, and let's not forget that while Android itself is open source, the Android on your phone right now is full of binary blobs for even the basic shit. Camera driver? Image processor? Fast charging? LTE modem? Fingerprint sensor? OLED calibration? most of them are binary blobs specific either to the chip/sensor manufacturer or the OEM who designed the phone as they consider this stuff proprietary IP they wish to keep secret from the other Android OEM competitors. Move to LineageOS and you risk loosing a lot of your phone's fancy features because of that.

I love my Androids but it's still a mess and the openness is slowly going away(since Android 9 we lost call recording capability which was a huge blow for my use case). At least we still have f-droid.