I can understand how people really want a higher level cap and I can certainly see how, in early access, leveling up might feel too fast. However as has been said before, this is by design. Not by design of Larian, mind you, but by design of DnD 5e.
For your convenience,
here is the experience table of the game on roll20.net.
As you will see, the amount of experience required for each level is not linear - it increases as you level up.
This is a bit misleading imo, as you have to consider that the experience you get from higher level monsters
also goes up. Not as quickly as exp required to level up does, sure, but enough that time to level up doesn't scale exactly with the experience table.
I agree that level 1 -> 2 and level 2 -> 3 should come fast, but not incomparably fast to other levels. Relating to tabletop, I've found that it feels about right to reach level 2 in 1-2 sessions and then level 3 in 3-4 more sessions, with later levels coming every ~4-8 sessions. In particular, level 2->3 should be significantly longer than 1->2.
Plus, you're managing an entire party, have access to weapon skills, any class can cast any scroll, everyone has bonus action shove and hide, and there are a decent amount of consumables. So there's still a lot to do even at level 2, unlike in tabletop where a martial character might be extremely limited in options.
