I think he mentioned acting out when he was younger. He'll get aggressive with people he's comfortable around. We saw this in the video where he's trying to fight his old roommate who called him a pedophile. His folks are getting up there in years. If he gets physical with them, they can't really defend themselves and they're old enough that they might have an issue recovering from a physical altercation.
Plus they probably have a relationship with their step-kids that is contingent on Lucas not being around. It wouldn't surprise me if his niece's parent's had an ultimatum basically stating "If he's around, we're not". Which would you choose? A fulfilling relationship with your grandkids or housing an obese unmedicated schizophrenic pedophile who may or may not get physical and definitely won't shut up about wanting to bone your grandkids.
As we saw during the FartBox Saga, giving him money doesn't end up with him making good choices with that money. It'd be like me giving my kid a grand and letting her go shopping. We'd end up with a mountain of Shopkins, cheap overpriced plastic toys and JoJo Siwa fashion accessories. In her defense, she's 6. With Lucas it wouldn't go towards clothes, a better apartment, etc. He'd blow it on manufacturing nonsensical board games like Cafeteria Uprising and/or board games that he'd never play because he's antisocial. Giving him money is just throwing good money after bad.
They tried housing him back in his earlier days, but that ended poorly. The apartment was messy to the point he was about to get evicted until his folks came and cleaned it up. He then started threatening people to bring him women like he did during the fartbox saga.
In short, unless Lucas underwent some dramatic changes he's beyond help and I think his parents, while likely sad about it, realize it.