Not really sure why, but my guess is that it's like that to stop abuse. As a S10 bless would give 22MR before anything else is considered, which could lead to a lot of SC's having 35+MR. That would take some getting through.
Easy enough to test as well. Just create a S4/6/8/10 pretender for any nation with native MR18 units (eg.Ashdod) and turn one into a prophet. You'll see -
Basic MR 18
+1/2/3/4 Bless
Current MR 18.
Would post a screen shot, but sadly don't have access to Dominions in work
