"It stores electric energy under very high pressure. It often explodes with little or no provocation." (Pokemon Red)
Don't have any limbs? Constantly on the verge of explosive death, which can happen for any or NO reason, which is also your signature move in combat? Well, you might just be an Electrode. Given the horrible nature of its existence, the idea that it could literally self-destruct out of nowhere while sitting still is probably a comfort it could be freed from its terrible excuse for a life at any moment. And, who knows, maybe it'll take down a Mr. Mime with it.
"A host-parasite pair in which the parasite mushroom has taken over the host bug. Prefers damp places." (Pokemon Red)
"When nothing's left to extract from the bug, the mushrooms on its back leave spores on the bug's egg." (Pokemon Crystal)
Parasect is controlled by an out-of-control mushroom parasite that has grown to enormous proportions, as well as by whatever trainer ends up catching the poor thing. That makes Parasect the slave of a slave. That's not a good position to be in. It has no control over its own existence, and whatever life-force it has left is being constantly sucked away by the huge fungus growing on its back. At least there's hope for future generations though, right? Wrong. The mushroom's spores infect the host's eggs as well, so even its poor children are doomed before they're even born.
Think of it as a reverse Mario where mushrooms seal you to a fate worse than death.