Often an acquisition of a company is for the set of customers. If I sell my lawn care business and then turn around and email all my former clients offering them lawn care via my new company, I’ve just undercut what I just sold.

Noncompete shouldn’t be so broad that I couldn’t move to another city and start a lawn care business there, but I shouldn’t be able to compete directly with the business I just sold using my insider information of that business.

Why would I buy a business if the person im buying it from is just going to create a new one to compete with me?

That is not even remotely the point OP is arguing.