Two problems:
- Locked bootloaders: if you're not given the option to unlock it, you can't boot anything other than the stock ROM
- Proprietary drivers: even if you can boot a custom OS, many Android phones have very unique and obscure hardware and the drivers for them are only distributed through their stock ROM.
The closest thing is LineageOS. If your phone doesn't support it, there's Generic System Images (GSI), but there's no guarantee of getting cellular service after installing a GSI due to proprietary drivers.
If you want something that just works, GrapheneOS is great, but you'll need an unlocked Google Pixel.
It's there to protect you from crimes of opportunity. Like if your car is locked, a thief could decide to pick the lock, smash the windows in, or find another victim, but they would have no second thoughts if your car were already unlocked. The password deters a casual hacker and buys you some time to notice and deal with anyone seriously trying to break in.
In an ideal case of disk encryption and a well-designed lock screen, the password forces a would-be intruder to either spend lots of time guessing it or shut down the computer, thereby discarding the encryption key from memory and thwarting the attack.