Noticed that there is no level 81 Achievement, but there is a level 50.

81 actually takes some time to get there and is worth the achievement points, curious as to why its not there? Not sure if this has been answered

Because achievements are made for casuals and not dedicated gamers.

I really wish there were more challenging ones in the game.

Wish there was a master-difficulty type achievement as well. Beating the MQ with Master on from the Start until Finish etc