Because?
Because remember when they were 20 muni?
During all that time, since release, till 10 muni cost they were STRONGER then current 3x.
I don't remember a single post complaining about old ppsh and if there was no complaining back then, then complaining now is not valid as well.
3ppsh allow cons to scale into late game and still are weaker then when they costed 20muni.
Test it more. If testing it more it turns out to overperform we could eventually suggest to give just buildings built information and not units built.
Which would make it even more useless then tank traps.
Can you really not see the stupidity of your suggestion?