The simple contiguous district system tends to have an anti-city bias.Here in PA we have great difficulty electing Republicans statewide - because of the cities - but it takes no great feat of gerrymandering to obtain a surprisingly strong Republican majority in the legislature and in the congressional representation.
The object of gerrymandering is to lose as few districts as possible, by means of concentrating opposition support in districts your opponent will win by lopsided margins. And the fact that Democrats cluster in the cities makes it easy and natural to draw districts which accomplish that for Republicans. So it doesnt look like a gerrymander with weird district lines when the Republicans do it, but if the Democrats shaped the districts in a way to just give themselves and even break it would look on a map like an obvious gerrymander.
Democrats NEVER do anything to give themselves an “even break”.