Why don't Arcane and the Galactic Time Guard stop all human suffering here on earth?

It just doesn't work that way. Changing time lines is very serious business so mostly you only change time lines to prevent cities from being nuked out of existence and the same for Countries or whole planets. You don't usually change everything to be living in some fairy tale existence that cannot be maintained in physical reality anyway.

Mostly you just try to keep the human race from going extinct here on Galactic National Park Earth. And you try to keep as many other living creatures on earth alive too as possible if you can.