Same reason why Lana’s been able to walk around with a chunk of it around her neck for the last ten years. It’s K+x=S (Kryptonite plus x-factor equals superpower). Electrical Scarecrow? K+big-ass meteor to the head. Bug Boy? K+insect bites. Freeze Kid? K+extreme cold. De-aged Guy? K+electricity. The Firey Coach? K+, um, firey temper? K+steam? He kinda cocks up the theory since there isn’t some specific event that triggers his power.
Speaking of which, where has Lana’s plot device, I mean necklace, been recently?