Of course people care what other people do, you're lying to yourself if you think otherwise. This is why people buy clears in the first place. This is why shiny rewards for countless hours of work exist. This is why rare mounts are popular. People just like showing off. And other people see the shiny things that the first person has, admire those things and care enough to actually want to go out and do the same. It's one of many reasons MMO's are popular, it's normal human behavior.
Being able to purchase a clear devalues the accomplishment.
But to look at it another way, why should carry runs be allowed? How does it enhance the gameplay? Would it really be a big loss if you couldn't buy Ultimate raid clears with crafter gil?