And what? Let government dictate how a business is run?
Why not? The whole concept of a "business" is nothing more than a fiat defined by the government.
There are all sorts of rules regarding businesses. One is that customers that owe you money have to eventually pay, and if they don't, you can run to the government for help. In this particular case, large employers have to treat their employees somewhat like human beings, and not just totally upend their families' lives with no warning.
Are employees required to continue working until I can find and train replacements?
Maybe if employees routinely quit en masse without warning, that would be an issue. But it's not an issue. How do we know? Because almost all businesses use an "at will" work policy if they are allowed. This means that being able to unceremoniously dump employees than is much more important to them than the risk of some employees suddenly quitting.