Butcher pet is pretty neat. Cow is just a 4-legged cow. Nothing special. The 3rd I think will be a Nercomancer exclusive pet, and right now I'm trying to collect the Cultist pages for the 2nd portrait. The first portrait is bleh.
Some people like the hunt game involved in attaining the cosmetics. What other hunt do you have in this game? Rift after rift, greater rift after greater rift, and zero end-game. Just like how people collected the unique blues back in the day (and some still do), like the Murlocket, Rakanishu's Blade, Wirt's Original Leg, The Clipper, Griswold's Worn Edge, etc. Some people who play like to collect things, and cosmetics are things to collect. Besides, every player is different. You may not care but others do (obviously). Most importantly, it's none of your business what other players want to do.