I think it's because it's holes in pickguards would be more of a custom item. Strat jacks and pickup rings are mass produced to a standard, alot of them made out of metal, so the driller only has to think about one hole to drill in the wood and not think about this mass produced item. Pickguards and plastic you have to think about holes in the pickguard AND the wood. Plus plastic can change based on temperature and moisture (just not as much as wood ....
I think this is a great idea, but just start out small.
Of course, I could be totally wrong, but in my carpentry days (renovating houses) and installing doors in schools, the home renovations were hard, because everything is custom, and at the school everything was mass produced, no custom holes, just matched up perfectly. And if it didn't, who cared, you just jammed it in.