Why do most/all electric and hybrid cars have regenerative braking while ICE cars don't? It's because, as the GP said, the battery would be quickly topped up and extra energy wasted. Only electric vehicles doing frequent stops/downhills can make productive use of regenerative braking.

Potential energy lost by a truck going 10 m (vertically) downhill: 40 000 kg * g * 10 m = 4e6 J.

Capacity of car starter battery: 40 Ah * 12V = 400 Wh = 1e6 J. Let's guess a truck battery has 4 times the capacity of a car battery, so 4e6 J to full charge.

That's just down one small hill and all subsequent braking energy for the entire trip is wasted.

To drive the point home, a Swiss company is working on a mining truck that will be a net generator of electricity - it goes up the mountain empty, but when it goes down it is full of heavy ore, and regenerative breaking charges the batteries more than was used to climb the mountain. The extra 10kWh can then be fed back into the grid.

https://arstechnica.com/cars/2017/09/this-cement-quarry-dump...

... Also because electric regenerative braking requires a battery and electric motor (for propulsion, the starter is useless for this). Once you fit regenerative braking into an ICE vehicle it becomes a hybrid vehicle, leaving no ICE vehicles with regenerative braking. :)

(I don't think you would call a vehicle with mechanical KERS system a "hybrid", although it technically is as there are multiple propulsion systems in the vehicle.)