"Why isn't gerrymandering illegal?"

[MineralMan]

7. The biggest problem is that states are in charge of elections within

their states. Congressional district elections are state elections. So redistricting after each census is done by the individual states, and every state does it in its own way.

In some states, like Minnesota, there are strict rules for redistricting that make it very difficult to do any real gerrymandering. In other states, the state legislature does the redistricting, and then it becomes a political football, with the party that controls the state legislature doing everything it can to gerrymander the districts to benefit that party.

Congress can't do much about redistricting, since there's nothing in the Constitution about it. By federal law, congressional districts must have the same population so they are all roughly the same. But that's it. Beyond that, it's up to each state to figure out, and it's unlikely to change.

The Constitution limits the federal government to just the things specified in the Constitution. That's slipped over the years, but elections are still a matter for state governments. Congress can say when the Presidential elections are held, but that's about it. In the Electoral College, Electors are chosen according to state rules.