I think it has to do with the math of level progression + how D&D defines tiers of play.
Math: I just looked at my EA save games and here's my levels by the hour.
0:00, level 1
0:40, level 2
2:30, level 3
8:00, level 4
16:00, level 5
Now this is a big if, but we assume 1.0 follows EA progression, and it's around 8 hours a level, it continues such:
24, L6
32, L7
40, L8
48, L9
56, L10
64, L11
72, L12
And they say the game can be finished within 75 hours if you beeline. That means just the last few hours of the game are max level.
There is some nuance to the xp system, some level-ups are slower than others (4 to 5, 10 to 11) and completionist play means spending extra time in Acts 1&2 where you're getting small xp awards relative to your level. So a completionist could actually take 100 odd hours to get to level 12.
D&D tiers of play states Tier 3 (levels 11-16) is saving the kingdom, i.e. the city-state of Baldur's Gate. Tier 4 is saving the world, which requires a drastically redrawn made and even more powerful antagonists.
