It's not necessarily that it's tricky to crack (it's certainly not easy, don't get me wrong), but that there's no point for a couple reasons that combine:

1. To crack a game you have to redo it any time there's a major release of the game, such as DLC/expansion/major bug fix. The reasons for this are numerous and outside scope. But it takes time.
2. Most crackers can only do so many games, so they often wait until most or all major additions are out.
3. Denuvo is expensive and operates on a yearly license
4. Most game studios only license Denuvo for those first few update cycles when they get the most sales and then remove it themselves because of the cost

That means many don't even bother trying to crack Denuvo because they just can wait it out. It's a resource balancing game on both sides.