Real life is hard, full of challenge and death. I don't want that in my game. I like my utopia where everyone lives forever, happily ever after. When I want challenge, I create something like an ambitious elder with eight toddlers, starting out with $100, attending school to better herself while working to survive. Or I create internal rules, like charging my Sim money if they leave the lot. Or I create a hotheaded loner who wants a million friends but yells at everyone. Simply creating a household with seven pets presents plenty of challenges! Sims spend all their time cleaning up and trying to keep the pets from running away!
I agree that more active careers would be nice. I really enjoy them.