Why the anticapitalist effort for free phones wouldn't base their system on a reliable and dirt cheap product instead of the branded icon of the enemy is beyond me.

Why has been explained time and time again. They're the most private and secure hardware, bar none.

https://grapheneos.org/faq#future-devices

Most OEMs don't even let you unlock the bootloader, so that instantly disqualifies like 98% of the market. ARM is not like x86. You can't just make a spin and then that spin basically just works on any old hardware, it has to be tailored to the hardware. And that takes time, money, and energy. Something GOS has limited amounts of.

Most cheap phones are cheap because they're subsidized by spyware.