Why are PC's so much more prone to technical problems than phones or tablets are?

Desktop and laptop PCs have an operating system model which allows software installations to modify the state of the machine.

This means that the device starts to change and mututate over time.  The installations of new apps, new drivers, new daemons and services start to accumulate.  This increasing complexity over time can lead to unforseen collisions and complications. Especually if some of the that software is misbehaving.

Phones, and especially  iOS was designed from the outset to prevent such cruft accumulating.  Apps and installations may not modify the OS. The application only has access to its own sandbox. It can't change the system. It can't change other apps.

This means that over time, the device does not get slowly worse.

There is a cost for this stablity in less flexibility.