The thing is, both sides do do it (heheh, doodoo) - when they're given the chance.
The nature of the Midwest is such that outside of Illinois and maybe Minnesota, none of these states will ever have a Dem trifecta drawing the lines for themselves. Wisconsin, Michigan, Pennsylvania, Ohio, Indiana, Florida (not Midwest but I'll lump it in here because it also applies) - the most you can ever hope for in these states is that they elect a Democratic governor who can force compromise maps, and that there's not a supermajority in the state legislature making their veto power a moot point.
Combine that with GOP dominance in the South, that means the Republicans are guaranteed partial to total control of redistricting for a huge swath of the country. The other parts are Dem strongholds which are either so strongly Dem and the states so small that it doesn't really make a difference (Northeast), or they have nonpartisan redistricting schemes that make it impossible to gerrymander (California, New York, New Jersey, Washington). On paper, that's a good thing, but it's a crippling handicap for all the blue states to go for this neat and tidy good governance crap while the red states completely do not give a shit.
Unless there's such a huge swing that we can pick up a bunch of these state legislatures (which are ALSO gerrymandered), the most we can probably do in many states post-2020 is neutral maps. Which is still a big upgrade from where we are now, but this is why it'd be nice for the Court to establish a national standard. Even if those swing states have fair maps, the deck is still going to be pretty heavily stacked against us thanks to states like Texas, Georgia and North Carolina having horrid GOP-drawn maps that at best will be struck down by like 2028 after the GOP has already won every state election under said maps.
