Severability (the ability to "sever" part of a contract, leaving the remainder intact so long as it's not fundamentally a change to the contract's terms) comes from constitutional law and was intended to prevent wholesale overturning of previous precedent with each new case. It protects both parties from squirreling out of an entire legal obligation on a technicality, or writing poison pills into a contract you know won't stand up to legal scrutiny.
If part of the contract is invalidated, they can't leverage it. If that part being invalidated changes the contract fundamentally, the entire contract is voided. What more do you want?
It seems like you're arguing for some sort of punitive response to authoring a bad contract? That seems like a pretty awful idea re: chilling effect on all legal/business relationship formation, and wouldn't that likely impact the weaker parties worse as they have less access to high-powered legal authors? That means that even negotiating wording changes to a contract becomes a liability nightmare for the negotiators, doesn't that make the potential liability burden even more lopsided against small actors sitting across the table from entire legal teams?
I guess I'm having trouble seeing how the world you're imagining wouldn't end up introducing bigger risk for weaker parties than the world we're already in.