People play video games for a variety of different reasons. Some do it to let off steam at the end of the week, others like to stay in the loop by keeping up with each new release, and some are addicted to the "ding" sound that trophies make.
Even though most of us don't go out of our way to 100% complete every single game we start, that feeling of earning a reward for completing a task is something that humans are just programmed to enjoy, and the ability to unlock funky costumes or additional modes is a crucial part of why games are so engaging.
Unfortunately, these rewards aren't always worth the effort. Sometimes, they're so difficult, annoying or time-consuming to get that they really aren't worth stressing out over, and in other cases, the reward itself might just suck.
Even worse is when you're awarded an item or power-up that would only have proven useful if you'd had it earlier in the game. After all, if you're good enough to slay the hardest boss, why do you then need an extra weapon upgrade?
To rewards like these, we say thanks... but no thanks. Give us something more worthwhile next time, and keep the lame cosmetics and pointless buffs to yourself.