All modern phones come with a locked bootloader. This is to protect the OS and the User data, preventing anyone with physical access to the devixe from reseting the device and installing their own software without permissikn. I had to unlock mine on my 1+ 7P before installing /e/OS. It might just be that carrier locked phones might need something extra to unlock them not sure.
Because there is practically no risk of damaging the device and bricking it. If you knew anything about the the distribution, it’s inhalation and android you’d know that the risk is almost nonexistent and reversible. I understand you don’t want to run any risk, sure, it’s a valid reason, but to damage the device is practically impossible just from installing another android variant.