This is true with ARM in general. There's no "standard Linux" to boot because every board needs its own device tree and set of core kernel modules for detecting important things like local storage. It's fairly intractable due to how different the hardware is.
I've heard this argumane before but that doesn't change the fact that some socs work out of the box and require no proprietary software or custom configs
Wow, I was sure Raspberry Pi were pretty good about mainline support, especially since multiple distros support the platform.
Software support is still very good compared to pretty much every other arm board.