@eniko This is deliberate. 5e progression is divided into tiers of play and 5th level (when wizards and other full casters get level 3 spells, like fireball and flying and hypnotic pattern and such) is the threshold of tier 2. Designing the game around those thresholds was done to let it work with the traditional D&D spell list and associated progression. Everyone is supposed to get a big bump in power at or around levels 5, 11 (wizards get 6th level spells), and 17 (9th level spells.)
Doing this lets players choose what tier they want to play at and makes it easier to scope adventures properly (eg. levels 1-4 are small-scale local stuff; at level 5-11 you move towards being regional heroes; at levels 11-16 you're focused on worldwide threats; and 17-20 is about facing cosmic threats, evil deities, etc.)