I think it was mostly that they were left with Tommy's dad who often pawned them off on the granddad. Just about everyone else had a job.
So as I've gotten older, I've started to notice that the antics that the babies were getting into would actually be horrifying from a parental standpoint. How many times did the parents leave their kids somewhere? I mean I always knew that Angelica's parents were bad with a neglectful mom and dad who spoils her, but damn. Leaving your kids overnight in a toy store? Ditching them with some guy you don't know for no reason? Leaving a mech unattended and easily accessible/operated by 1 year olds? Stranded in the wild twice?
Edit: I knew a tot that put an airhead in his diaper because of Rugrats...nasty af.