I wouldn’t be surprised if there are a few global communist agrarian worlds in the lore that just quietly have relatively happy humans doing their thing with just the yearly Imperial tax collectors swooping in and swiftly fucking off. So long as there are no mutants and no chaos worshipping the Imperium would probably never bother checking in.

Nooooo, an environment that's a little miserable and uncomfortable is necessary, just look at Rophanon.

It was an idyllic planet, then the Imperium found out and they were like 'Hey, maybe we should turn this into our own personal four seasons hotel', and so the administratum (name?) turned the planet into their own resort and made the populace be subservient to them. The people were like 'Ey, life's gotten kind of harder and less rewarding since these guys came here' and organized a violent revolt. Unfortunately for them a space marine leader (Inquisitor? Commisar? I only saw a breakdown of what happened, so I've no idea what they were specifically) was taking his break there too, and so the planet was stuck in violent war for several years. A tyranid hive fleet was passing near the system, and so the commander decided to entice the hive fleet into finding the planet (because if the imperium can't beat the native populace, then no one was going to have this planet) and then took his troops off-planet and after the tyranids spent some time planetside causing havoc and killing tons of people, the commander ordered an exterminatus to utterly destroy the planet.

And of course the moral failings of the imperium, and how they exploited and then genocided their own people was lost on fascist fans of the game, it was just a cool bit of world building.