Because historically, Republicans haven't concerned themselves with worrying about things like "fairness" and "the will of the people". Democrats are only really starting to fight back now (not that there hasn't been gerrymandering in favor of Democrats in the past) because the Republicans have gotten so egregious with it recently.

As for who gave them the right? The Supreme Court decided that redrawing districts for purely partisan, political reasons is apparently A-OK. So now, politicians can choose their voters, rather than voters choosing their representatives.

So who redraws them and who gave them the right?

The states have the right to control the size and shape of their districts and is pretty much always done after the census which occurs every 10 years. The redistricting is supposed to be done to account for changes in population, but as you can see now Republicans pushed for a mid decade redistricting for reasons purely about political power, it has nothing to do with representation of people where the total population has changed.