For odd historical reasons, when an employer purchases health insurance for an employee, they get a huge tax deduction. So having employees buy their own is significantly more expensive, even for the exact same coverage. In addition, in most states in the US, the individual insurance market is a disaster: prices are outrageous, there is no common market where you can look up prices so you have to find and contact individual carriers, you can be denied coverage for just about anything, coverage standards are non-existent so there's no way to compare plans based on price, etc.

The end result is that many developers aren't willing to take jobs where health insurance isn't included.