Thanks, yeah that makes complete sense. Plus I would imagine the more energy density you need, the heavier your enclosure has to be, always partially negating the benefit!
This is why it's a bit hilarious to see how upset people are about lithium batteries occasionally exploding. That's... sort of the point! They've been optimised until the safety margin (=weight) is a low as possible. You want safe batteries? Carry around something the size and weight of a brick!
Ditto with all possible kinetic energy recovery system (KERS) designs. You can have weight-efficient or safe, but not both.
Putting armor, heat sinks, etc. on them does make them safer. Adding battery safety circuitry (charge control, overcurrent protection) also helps. It also adds weight. Unarmored lipo batteries are notoriously dangerous for a reason, and it’s not the electrolyte.