Somewhere around where the slippery slope fallacy starts.
Snark aside, there's no real downside to mandating something in an employment contract that is probably already in most employment contracts anyways. This catches the stragglers.
Why don't governments produce standard draft employment contacts? They already do so for many things (e.g. buying/renting property).
Employer could ignore the standard or modify it and the employee could still amend it but it would help small companies get this sort of thing right and help with protecting both parties (which is the point of an employment contract).
In practice, lawyers will produce/modify standard contracts for extremely cheap, and as a bonus, if something goes wrong, you now have a relationship with a lawyer. If you're really cheap, in my country, you can literally go to the local book store and pick a model contract off the shelf.
I don't think this is a problem that needs solving by the Government, since these contracts are essentially a commodity and have wound up being priced as such.
Snark aside, there's no real downside to mandating something in an employment contract that is probably already in most employment contracts anyways. This catches the stragglers.