I think it’s because it’s been tacitly consented to all this time by literally every staff and their masters turn a blind eye as well. You know what they say, the servants’ attitudes reflect their masters, so since their masters don’t even treat Penelope as one of them, the servants had no reason to either. The situation would keep deteriorating as it starts from a small action, seeing that it begets no consequence, would spur on more and eventually spirals to now where she’s hardly treated any better than even the servants. I believe Penelope also serve as an object of stress relief and twisted sadism for the servants. No wonder Penelope went crazy. Honestly, if it was me, I would have brandished a knife and chopped them all to death.
I don't understand how some maids have the gall to treat a member of the family they work for badly when they don't even have someone backing them up