Why doesn't homebrew gear use universal sizes for taps and pipework?

You go to a plumbers merchants and you know what you can do ie, reduce from 10mm to 15mm, 22mm to 15mm the choices are endless.
I spent 2 hours today trying to find some pipe for my boilers nylon tap and when I found it, it needs reducing to fit the spinny sparger...can I get a reducer.
I know I can acetone weld plastic but I shouldn't be even having to think of this.
I'm very tempted to rip the HB gear out and outfit with plumbing gear as I know a few of you more experienced guys already use.
Grrrrrrrr