Explain Like I'm Five

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.

So who redraws them and who gave them the right?

Who ever is in power at the time unfortunately. California and Texas have both been in the news over redrawing their districts recently to give themselves an advantage. Each side gerrymanders so they won’t agree to abolish it.

And why does it always seem Republicans always wanting to do it?

There’s a bunch of factors but if I had to choose one reason, it boils down to low voter turnout. In the example I gave above, imagine that the 100 voters in each district was less than half of the eligible registered voters in each district.

Gerrymandering doesn’t mean you automatically win, it just means you have an advantage. If a district has a lot of swing voters or higher voter turnout than usual, it can work against the party in power that redrew the lines.