>>96876424
For most of the 10,000 years in between the Horus Heresy and now, the Necrons were mostly asleep, the Tyranids had not arrived, and Chaos was mostly contained to the Eye of Terror. There are entire Millenia during this period where basically nothing of note happens other than the founding of more space marine chapters. Were there a few major events, sure. But there were millenia between these events.

The Imperium is playing on easy mode. The Tau have to fight for every inch of territory against a massive hostile empire, while the Imperium was able to conquer a million planets that almost universally had no FTL and no forces beyond whatever was local to their planet, and then afterwards had thousands and thousands of years to build up and reinforce their rule with relatively few interruptions.