Sure, and you'll find most places which don't have "at will" likewise do not permit employees to just arbitrarily down tools and walk away from the job without consequences.
Our systems administrators had to give 90 days notice when they more or less simultaneously quit. That was enough time to identify any important work that hadn't been documented, start hiring replacements, figure out what our plans were for the interim and so on.
But if your business will collapse without just one person you're in trouble anyway due to Bus Factor. That one person might be kidnapped, have a mental breakdown or indeed get hit by an actual bus.