Breach of trust... Lowers the reputation of whoever rescinds. If a company does this they get a bad rep and fewer applicants. A employee does this and it might get around costing them job offers elsewhere.
The tech giants (or other corps) have spread the fantasy that if you piss one of them off then everyone up there will know and black ball you. Basically unless you don't do anything super illegal the odds of fucking over your employer coming back to haunt you are relatively small. But the fear that it will keeps people in the order the corps want.
But who would the reverse be true? It’s not as if there’s a database of employees who have signed a contract but didn’t show up...