We've been hit by this problem as well in our local office (engineering/consulting). We have only have budget to fill one (1) position and we've had three (3) people ghost us after signing an offer letter (salary job with good benefits), or tell us the day they negotiated their start date that they've accepted a job somewhere else instead. It's been frustrating because we kind of have to stop looking for someone to fill that role when a candidate accepts the offer, and then we have to start over once they ghost.
It's a real bummer because we'll move schedules around to have someone available for training, and then... nothing. It's been frustrating for a lot of people. We had another person leave the company from a field office about 10 hours away because we couldn't replace a person who left on their team fast enough. They were in a position where they were doing the job of two people for months as we went through the ghost/reneg train with 5-6 people, and we were constantly flying in people from other offices across the country to try and fill in for them when they couldn't keep up, he quit last week (who wouldn't?), and now it's worse than ever. We finally just hired a person to help him and now it's training a new person and trying to fill another position for that team. It's brutal out there.
We offer good pay and benefits, it's just a rough market. I wonder if that signed offer letter is a "pay me $10k more than this" card.