If I had to guess, I think it is because hunter attire/tack are workmenlike, whereas in dressage, with its military roots, more ornate attire/tack that echoes a military dress uniform is the norm.
I was shocked at how cheap my dressage debut was compared to my child’s hunter shows! My coach was an eventer. And they don’t really go to things with you. At first, I wanted to whine “But, but, I’m a schlocky AA! I need adult supervision!!” (To which she no doubt would’ve pointed out that I’m older than her in her polite, Canadian way.) Leased horse was much more accomplished at dressage than me. I took the advice of his mom to register for 2 tests as he might go round like a giraffe in the first.
We hitched a ride with another boarder. Horse was excited about his triumphant return to the dressage court after 5+ years & somewhat irritated we’d entered Intro & not PSG, but mostly held it together for his hapless, green bean human. We pinned in both tests, no one died, & the whole affair cost 1/3 of what a hunter schooling show would be.