The original Baldurs Gate complete with expansion got you to level 7 I think? DnD tends to be a slower levelling experience than most video games, and that gives you more time to actually roleplay. To develop a character. That said, I think Larian have now committed to a higher cap, but not stated how high. I wouldn't expect much more than 12.
Apart from the commercial reason to leave room for an expansion/sequel to 20 (BG2 allowed you to import your character from the end of BG1 which was an amazing feature), balancing the extra feats, spells, multiclass options is a lot of work, and gets more complex with every level as interactions become more intricate. At the end of the day they want to get the game released this side of 2023.
