There's just not that much power available for regen. I've done a lot of dicking around with E-scooters. Even at their low speeds with lots of stop and go, regen is less than 10%.

It's only used because dumping excess energy back to batteries is cheaper than including brake hardware. The math may work out the same for EV's. Regen just to decrease the cost of brakes rather than increase range significantly. In the e-scooter world, the cheap ones use regen and more expensive models have traditional disc brakes.

Air resistance burns a ton of energy at any speeds over 20mph.

How much RPM were you getting at your scooter, and what was the vehicle mass?

You say 5% at most... The difference in between, say, 5% and 10%, a bad and good regen is huge.

The RPM's are huge. Like 16k, because smaller motors are lighter for same power. Mass is generally 40lb + rider.

The RPM doesn't matter much though. Regen efficiency is around 80% from wheel to battery. With cars you get much less regen because you lose tons of energy to air at the speeds they travel.