Because the core reason is about control. They don't want users to have the option or freedom to install an OS of their choice because it could hurt their "precious" revenue

A lot of carrier phones come with carrier preinstalled apps. And it allows for sim locking, keeping you trapped with them or other carriers on the same network. Or at least that’s how it has been, back in the day, when sim-locking still was legal in the EU. Now, phones are the same, whether they come from your carrier or retail.

Their stated reason is to mitigate theft by preventing removal of software that binds the device to the network and account, and to protect their network by blocking paths to custom roms including potentially custom radio firmware.

The real reason is likely a blend of protecting leased devices for resale value, keeping people from removing "sponsored" apps or ones that make them money, and distrust of users ability to not get tricked into abject stupid choices.

You don't think they care that you're attempting to circumvent their data collection on you?

They care because some users don’t actually own their phones and the carrier wants to keep strings attached, or they want to impose artificial software restrictions like preventing or limiting hotspot data.

Even when none of those conditions apply, you still often must deal with the locked boot loader. It’s BS.