Why wouldn't the
float valve be as good in a motorcycle as in a car or lawnmower?

It would, however in a car the gas is pumped from below so once the
pressure is gone, no leak.
A lawn mower is gravity feed (except those with a diaphragm pump built
in), but usually just barely, the tank is seldom much above the carb. I
motorcycle is the perfect storm; with the tank well above the carb and
fairly large fuel lines.