Why are bicycle gear ratios computed as front/rear and not the opposite?
I think it's simpler than what many think.
Car and motorcycle drivetrain is a reductor: their ratio (driver to wheel) &lt; 1 in most cases. Bicycles are the opposite: the wheel normally turns faster than the cranks, and the drivetrain is a multiplier.
It is somehow easier to remember and 'feel' ratios that are greater than 1: compare 4.3, 3.7, 2.9 with 0.23, 0.27, 0.34. So that's how it's done.
In the end, this is the same argument: tradition. But at least this is an attempt to explain how such tradition came about.
Besides, cyclists rarely operate ratios per se. We may fancy with 'proper' units like gain ratios etc. mentioned in other answers, when selecting a new cassette, but in practice we just know it by the teeth :) Tell me any standard ratio expressed in teeth (say, 39/17), and I'll immediately say at which speed I will use it and how 'hard' it is. Yet I don't even know what ratio it is without a calculator.