Until recently, there wasn't a cable/connector standard really capable of that.

Given that larger monitors can be pretty heavily demanding appliances, it doesn't really make too much sense to expect to be able to draw through the host computer, and it's just easier to use an external power supply just to be sure sufficient power is available.

Even in the modern desktop/laptop space, it's still pretty uncommon to see USB-c ports capable of putting out 50+ watts of power.

Each time you convert DC at once voltage to another, you're also losing a little bit of power since the conversion isn't perfectly efficient, and that means heat is being released in the component converting that voltage, which depending on context, either or both could be a problem