As I understand it, in states where employment is "at will", you can form a written agreement with your employer, but most arrangements are at will by the mutual choice of the parties. It might work out like that in marriage too, if you gave people the choice - maybe people would stick with the current system because that's what they like! Of course, the fact that you're unwilling to give people the choice suggests that you're not too confident of that result.

Furthermore, this argument requires that you believe that in states where such an agreement is illegal (and I suspect, considering the restrictions on non-compete clauses, that there are such states) you're not free to hire someone for a job.

It is certainly true that there are a large number of unconscionable restrictions in the labour market, from rent-seeking credentialism to bogus cartelization and onwards. It is perhaps hyperbole to say that you're not free to hire someone for a job, because at least some terms will be enforceable, but I would certainly understand where someone was coming from if they said that.