I don't think that tying level cap to difficulty would be viable. Levels come with specific increases in power and ability that they would have to figure out how to make viable. Then Larian would have to go through and adjust every combat in very minute ways, since higher level cap means enemies will have more abilities and such corresponding to the included levels as well. If they go that far, then it's genuinely more reasonable to just leave the level at maximum possible and adjust from there.
