I you run GrapheneOS, it is an open source platform built on top of AOSP (the Android Open Source Project). Part of the security model is that you don't run as root. I am an advanced user and I don't want to run as root on my phone, I am happy with GrapheneOS as it is distributed.
Now if you want to be root, you can install an OS that allows you to be root. Just like I unlocked my bootloader, installed GrapheneOS and relocked my bootloader, you can do that and install whatever you please. I will keep using GrapheneOS because that is the most secure OS I can find for my phone.
The problem, IMO, is not that "some OS are opinionated and don't give you root access while other OSes do give you root access". The problem is that on many phones, you are not free to install the goddam OS you want (e.g. because you can't unlock or relock the bootloader).
The wiping is a security feature: if someone installs a new random system ("random" being defined as "not signed by the same entity"), then they can modify the system in order to attack it. The whole secure boot idea is worthless if you allow that.
Note that secure boot doesn't become worthless just because you can flash something different. The TPM should notice SB is turned off, and should refuse to decrypt, but there should be a way for the user to back up the key and use it to recover the data later.
Mobile platforms and developers solely supporting their attestation should allow other forms of self attestation that the users want. I don't think I'm confused about anything, and I don't need to muddy the definition of security or root to argue having control over my data and apps on platforms I want to use.