Because they are essentially still race bikes.

If you want rack mounts get a touring bike.

A bike company doesn't want to imply that you can load up their carbon with a heavy load. It will just lead to overloadin and breaking.

There are often substantial geometry differences between race and endurance bikes from the same manufacturers. They are all road bikes, but not all race bikes.

Why would someone want the lackluster ride qualities, especially unloaded, of a touring bike that they don't plan on using for heavy touring? Also, there are lots of bikes that are not remotely touring bikes that come with at least rear rack mounts. We're potentially talking about the ability to throw a 1 1/4lb rack and an 8L trunk bag on a bike here or even bungee strap a laptop or book bag, not necessarily stuffed 25L Ortlieb panniers. I shouldn't need to buy a Surly LHT for that.

Yet there are many carbon gravel bikes that have rack mounts. Other than the ability to take very wide tires (and pretty much always having disc brakes), there is often not much difference between a gravel bike and an endurance bike. So rack mounts are good for one and not the other? That doesn't really compute.

Most endurance bikes share a lot more in common with a race bike than a touring bike.
The difference in geometry is fairly subtle.
Racks can easily be added to a race bike with some simple modifications. No need to account for them in the factory for either geometry when they are best suited for touring bikes.

Not if the manufacturer is taking the endurance class seriously it isn't. You will generally see a noteworthy difference in headtube length and in top tube angle, leading to a pretty big difference in average rider position over longer distances. What seems like a subtle difference on a quick test ride around an LBS can and should be far more than that after 100 miles. Also, endurance bikes usually also come with more tire clearance than their "race/performance" brethren. There can sometimes be gearing differences as well.

Or, better yet, manufacturers could just give their customers the option of adding natively what many of them will add anyway with workarounds, which is my point. Many buyers want a road bike that they can also use for commuter duty and they may want the practicality that comes with a rear rack. Most people just get to buy the one bike that checks all their boxes. I feel very safe in stating as fact that the substantial majority of rear racks out there never have and never will have panniers attached to them so they are certainly not "best suited for touring bikes". Why should buyers have to sacrifice versatility?

If this issue was limited only to the very high end models, it wouldn't puzzle me, but it isn't.