Why does Android have to kill processes?

Hey,

I'm the owner of a Nexus 9 and while thinking once again about it's memory problems I started to wonder:

Why is it that Android has to kill apps when running out of memory, yet PCs can keep running all the apps they want even if their combined memory usage exceeds the PC's RAM?

I have never seen a PC kill a program because of low memory, they just become slower.

Thanks,
Niko