This very question has asked quite a bit...
BG3 makes use of the DnD 5e rule set and once you past level 12 some of the spells become over powered and would be very difficult to implement. This is a limitation of doing a CRPG as there is no live DM to make judgement calls that would help balance these spells. As such going to higher levels create a scenario where they either have to lock down a lot of spells or develop the single most sophisticated AI in the history of computer gaming.
Also as with traditional ARPG design the game has a specific end point, the developers worked around this at level 12 based on the issues mentioned with the 5e rule set.
I understand your argument DYNIA that this is a CRPG and not DnD Table Top. However the CRPG has been always designed to be based on the Table Top experience and so make use of a variant of those rules and thus must work within those limitations.
