Because of all the little things they don't have: Keyboard and mouse, office suit. command line, etc. Sure you could probably solve all of those problems with enough time and money but you'd end up with something that's neither the same as the original console nor cheaper and easier to maintain than a mid-range laptop.
This is what general computing is about these days. Most consumer devices these days are powerful enough to do most general computing tasks. Even watches! But it's all the additional stuff you need to do to expose that power that prevents it from being a good general purpose device.
Whereas consoles are can be well suited in other domains outside of gaming even without installing additional peripherals nor hacking the OS.
