Ha! Good point... but that doesn't mean that setup works well for bikes either. Physics is still physics.

One pipe per cylinder, with no common collector, is tunable for one specific rpm very precisely. Just fine for a top fuel dragster that only needs to make power running flat out.

All cyls into one (like the iPd and others) is tunable for a wider range of rpm, typically used for max HP at the power peak. That does not produce desireable characteristics for other than high-rpm use OR midrange use, but not efficiently for both.

4-2-1 headers (for 4 or 8 cyl motors), or (3-1 + 3-1)-1 (for 6 cyl) is tunable over a wide range of rpm. This is desireable even for race cars. See what the NASCAR setups are these days... or F1, for that matter.