Every android phone I've owned can install apks just by opening them in the file manager. Not all Android phones allow this, but many do.

On top of that many brands (asus, oneplus, even some Chinese brands) let you unlock the bootloader with official tools. Using this you can take full control.

So it's far from 'none'. I recently bought a zenfone 6 because it satisfies this criterion. Asus is even sending devices to alternative os developers to help them out. So we can choose to not support walled gardens, and it's not even hard. It's a question of getting people to value software freedom higher.

Oh yeah, Android is also completely broken by design, I'm not advocating for the way it works there either.

No, it's not. It's not perfect, nor is it trivially easy, but it is possible to side-load apps on an Android phone.

It should be something the common user can do, right now you need to go to enable some obscure and scary sounding settings to enable that, that's unreasonable.

It doesn't matter if it's "unreasonable". The key is that it's possible. It's not possible at all on Apple devices, so Android is superior in this regard.