DnD (especially 5e) is a bit different this way. Your reward in the game is level progression. Loot and money are supposed to mean very little but the next level is everything. Combine that with the excessive amount of exp we get (which, once again, is excessive only if you do every quest there is, which not everyone will do, hence, excess) and the question is kind of logical - I have the means, I want new abilities coming with the next level, why am I denied them?
Personally have no problem with the decision. DnD is notoriously unbalanced past lvl12 (not many tables go past that number, very few official modules do too). But the question is still understandable.
