Evolution doesn't require each individual to live to a ripe old age when they can successfully spread their genes by producing litters of 4-6 babies up to three times per year, starting before they're even one year old.

Even if they slowly poison themselves and die at five years old they'll have plenty of genetic descendants that might also think poison tastes good.

Or they'll run into plants that arent native to their ancestors environments.

Or they have a deficiency for something so they're compelled to eat plants they might not had they not lacked a nutrient.

Or they're just menaces, munching up the neighbours flower bed for kicks.