Its pure cosmetics though...

I can assure you a lot of people arent happy with this, but here are a few answers:
1) Because they can - because people do pay that amount of money
2) Because they dont sell gamebreaking stuff - you can play the game perfectly fine without a fat cat mount or a specific emote
3) Because the mogstation-money is apparently the only money FFXIV makes that goes directly back into the game - our sub-money is used to fund other SE-projects aswell.
4) Everything that is in the mogstation is specifically being made for the mogstation. Nothing gets taken out of the game - we wouldnt have gotten something like the fat cat etc. if it wasnt for the mogstation in the first place.

You also dont need to feel bad for people who choose to spend their money on something they enjoy - may that be a mount, a box of expensive chocolates or anything else luxurious. Its their free choice to do with their money as they please and use it to buy something that gives them joy. Seeing how certain well requested mounts are locked behind achievements, some people might actually prefer it to just being able to spend a few dollars - instead of having to hunt down 2000 S-ranks.

If those mounts arent worththe money to you: dont buy them and stick with the things you get within the game.