True, but Google's own implementation of Android in the Pixel does allow you to re sign the bootloader so you can install your own OS and securely boot to it. And "Android" isn't all the same. It's a broad and shared ecosystem.
However, doing this and using a custom ROM will flat out disable SIM services menu and related functionality, which I need for my embedded e-Signature in my SIM card.
Also, we have a gatekeeper called Google Play Services. In other words, even if you have a custom ROM, you need that all-encompassing framework to have nice things.
Clients may differ, but the cloud platform and the connector which enables all these "Android" things is closed source, and controlled by Google.
Or you could have the open source microG and miss out only on Google Pay (and some bank apps that wanna validate SafetyNet for some reason). If you disable FCM / push notification support, your phone won't even need to interact with Google servers at all.
For low budget folks, a lot of Xiaomi phones are supported. You'll have to sign up for their account and wait for a few days, but otherwise the process is pretty straightforward. E. g. this is for my current phone: https://wiki.lineageos.org/devices/ginkgo/install
