“Earning” it isn’t important to many people. There are “things” they want, but the process to obtain said “things” is unpleasant and/or too difficult for them to accomplish. In some cases these “things” are time sensitive (that’s a whole other problem imo), such as mounts that can’t be acquired once a new expansion launches.
How someone gets the “things” they want is, more often than not, unimportant so long as they get their “things”.
In the case of gear being obsolete, I’d agree that it’s silly to pay for gear carries in regards to player power, but if it’s for aesthetics then I can understand why someone would. Transmog lasts as long as WoW lasts. So if you want that mythic plate set from Sanctum of Domination, but you don’t raid anything higher than LFR (if you raid at all), but you have a ton of gold, then paying for a carry to help complete the set for its visual aesthetics is understandable to me.
So why not just play the game and enjoy yourself?
That would require that aspect of the game to be enjoyable, and what is or isn’t enjoyable is entirely subjective. You might enjoy challenging content and despise cakewalk content. Someone else might enjoy cakewalk content and despise challenging content.
Why not experience the game yourself and not treat it like it’s some job that needs to be cheated by having others do it for you?
Because depending on the content required for the “thing” you want, the game oft times feels like it IS a “job” and paying for a carry is far preferable to getting the “thing” than jumping through the flaming hoops required to get it “legitimately.”
Personally, I’ve ended up snubbing content I don’t like despite the rewards, often due to obvious time gating or “intended to waste my time” design. The only case where I pay for carries are those above mentioned time sensitive mounts that go away when a new expansion launches. And I only pay for those carries if my guild’s raid group doesn’t offer to carry me through it.
I stopped raiding in MoP beyond the odd LFR to clear story quests and see the story play out, but if one of those time sensitive mounts appeals to me, I’m going to get it in the most expedient and least painful way possible. And that typically means a carry. Any other mount drop I can always wait until the point where I can solo the raid to get it.