If you're wondering why Diablo 4 has a horse for sale with $65 of premium currency stuck to it, let me introduce you to 'price anchoring'

pcgamer.com:

If you've looked at the Diablo 4 store and wondered why you can't buy something without it being part of a bundle, I regret to inform you that it's super on purpose. I talked about this earlier in the week, regarding a set of class-locked portal recolours going for $20 (with a mandatory $10 purchase of 1,000 platinum). Turns out, this happens a lot.

Exhibit A: the Vitreous Scourge bundle—which is a horse that costs $65, kinda. The mount is one of several bundles released for Diablo 4 over the course of its lifespan that let players cash-in for exclusive cosmetics, plus a premium currency reward. 

In this case, the Vitreous Scourge is technically—with a hefty asterisk next to that word—a good deal. 7,000 platinum is the equivalent of around 65-68 dollars, as per the platinum prices on the Battle.net store. You're getting a slight price cut on premium currency and free horse armour—horse. I meant horse. 

So what's the problem? Well—it does suck that you can't just buy the thing on its own. Ideally you'd also be able to unlock it in-game, too—but I'll grumpily admit you've got to grease the wheels somehow in our modern age of microtransaction hell. The real issue at hand is that there are buying options being withheld in the name of marketing psychology.

The «anchoring effect» refers to our tendency to see something as more favourable or likely as long as we get to compare it to an «anchor» somewhere. This anchor can be completely irrelevant—for example, in one study, researchers found that the presence of a wheel of fortune had an influence on participants' guesses as to «the percentage of African countries in the United Nations.» If the wheel landed on a lower number, the average guess was lower, and so on.

In marketing, this principle is applied in the form of «price anchoring.» Do a quick Google search, and you'll find reams of guides, blogs, and explainers designed to help you "play on consumer's biases". In essence, the prospect of shelling out $65 dollars on