It was explained many times in other discussions, especially at TWC. The goal is to keep WRE alive when they are run by AI. And actually it seems to be the best balance between too large and too small garrison for both ERE and WRE.
Just to remind all sceptics - historically WRE kept intact unitl 437 AD (until 423 AD it was real, after - partially or episodically). Yeah, part of Aquitania was in hands of Goths and west-north Hispania at Suebi's, but both were real foederati of Rome until death of Honorius. Honorius forced them to subject after 412 AD. The only reastless territory was small part of southern Spain, where "Vandals and Alans" tried to resist attacks of Suebi and Goths.