During the previous patches when you were creating multiplayer game if I'm not mistaken there was an option to choose what players level the server should be and the last item in the list was level 25+
I don't know where you people get it that the cap level would be 10? I said it several times here in the forum that in Grymforge you could be around 7-8-9 level and no one believed me. If we have 3 acts then I guess the max level would be around 25 indeed. Which is something okay, because we are talking about "normal" difficulty here.
That's almost certainly leftovers from the DOS2 framework they built BG3 upon. See @Rag's post.
In addition, leveling up isn't linear in D&D 5e. Let's say you could get to level 8 at the end of Act 1. This would imply level ~13-14 at the end of Act 2 and level ~17-19 at the end of Act 3. Not 16 and 24 respectively.
Also, honestly it'd be very difficult to balance a game for level 13+ D&D 5e play. Characters at these levels just get so overwhelmingly powerful and 5e isn't balanced for that level of play. A DM can adjust things accordingly to the party's strengths/weaknesses, but a crpg would be static. Plus then Larian would have to code in all of the high level spells and abilities, which is much more work.