It's not just the supply introducing harmonics and deformed power. Consumers do too, and every single rectifier plugged into the network contributes to that.

Think of every phone charger, every laptop charger, every device that uses a rectifier to power its electronics essentially draws non-sine current. There are standards for these devices of course, but it's the supply that has to compensate.

Add on top of that the fact that AC carries reactive power as well, which means that every asynchronous motor plugged in the system will further destabilize it, and you have quite a chore on your hands!

It's honestly a marvel of engineering that your supply is as close to a pure sine as it is :P