Think outside the box for a minute. It isn't just about them. If a member of their family gets sick, it could cause them to want out. Other types of family difficulties can happen. The kind that says they can't stay in Green Bay, as an example, while their family has problems in let's say Texas. So many things that can cause them to lose focus.
As far as their contracts, I can just picture an organization telling a guy he can't leave when his wife living in Georgia has the virus, and is going into hospital. Imagine how that would play out in the press, and the reaction from players.
Teams just need to expect that some players will leave for various reasons, and go on with the season. Others of course will be out because of them getting the virus, or being quarantined. Imagine going into quarantine on Friday, and not being eligible to play on Sunday. Gonna happen.