I have no idea what problem a flywheel would be trying to solve when you already have electric motors with full torque at any speed, putting that aside.
The clutch on a dirtbike is not used like the clutch on other things. They can be shifted without the clutch, part how their sequential transmissions are built, part the friction between the wheel and dirt being so low. You go through the whole range of power and speed back and forth so rapidly you don't waste time on the clutch most of the time, unless you're stopping. The clutch is mainly used to feather power output.
Dirtbikes (racing anyway) have too much power, that's how they function, it's like riding on a chainsaw stuck in the dirt. You never have full traction, the bike is always revved in the peak RPM for maximum power output. You stomp through gears to control speed, you use the clutch to control power, independently.. when shifting, the dirt is your clutch, it eats that power/speed difference. All that to keep maximum torque available a finger flick away.
So edirtbikes, similarly have stupefying power to weight ratios, like any bike at max power they will just cartwheel. I haven't tried one, but from what I understand they map power output to throttle position on a curve, and that is just weird to me. They have buttons to change modes that remap the throttle curve, but it doesn't change the fact that we just don't use the throttle that way on a dirtbike.
Functionally, there's no need to pin the electric throttle and vary power with a lever, that was part of keeping the engine at a constant high rpm, the power band. But, no matter how you map the throttle, even in some race mode, it still has to be low power at the bottom and what full everywhere else? That's still asking riders to use the throttle entirely for power management and it's not the same as the finger flick it takes on the clutch.
TLDR: in motocross, the clutch is how you control power output, keeping that form of input on high performance e dirtbikes makes a lot of sense.