Why does the battery drain so fast?

The lightsabers are never truly electronically “off”– if they were, then you wouldn’t hear kyber or blade insertion sounds when the switch is “off”, nor could the hilt scan your crystal and change the chamber color without the blade being on. Really, the switch only turns the lightsaber blade effect and sounds. I believe this was an intention design decision for immersion’s sake, to try to replicate how a real lightsaber would act in Canon. This has the side effect of draining more power over time, especially if you leave the blade connected/inserted in the saber even when it’s “off”.