Because a "tab" is a known quantity?
Back in the day you kept track of the picture on the blotter because experience suggested that it might take 3 hits of "globe" to get the job done but if it was "orange sunshine" you really only needed one. And that windowpane? You either got nothing or you lost track of the next 36 hours completely.
Fixed dilution makes sense if you know what you're starting with, but my experience was you didn't really until you had sampled the batch a few times to figure it out.
I read of guys into powdered drugs with good lab skills who test and refine everything they buy so they can get the dose right, but that's almost practical with stuff dosed in the 10-20 mg range. At the microgram range? You'd need a decent starting quantity and a mass spectrometer.