Because a smartphone is locked down by default. I'm sure there's lots you can't just easily do with it.
The development experience of something like Android itself seems super unfriendly for beginners (as someone who is admittedly a beginner to it)
The Raspberry Pi is also somewhat locked down compared to most competing devices. But yeah phones are even more. :)
There is a lot of AI-aided effort going on "hacking" the bootloaders. Hopefully someone will figure out how to unlock them.
It's not as locked down as you might think. I was surprised how easy it was to root a couple of devices, one an old pixel and one a pretty new one
It's definitely a bit odd, the Android environment, but I didn't feel blocked on it. Also ended up unrooting one because termux and the built in debian VM on Android now give everything I need
Termux + X11+ XFCE is surprisingly nice on a vanilla phone