You'd be surprised how much some people are willing to pay for fake bragging rights.

Buying a Ultimate win gives them an excuse to lord it over everyone else how much better they are, it goes roughly like "HAHA! I cleared Ultimate (Even though i actually didn't) That makes me awesome and better than you!"

Certain people will pay any price in order to look good.

And then the rest of the playerbase has to put up with their unjustified ego-tripping.
Even more obnoxius when they fail even basic mechanics in low tier content like 4-man dungeons or story trials and then blame everyone else because "I cleared Ultimate! I cannot fail! The rest of you noobs must be the ones at fault!"