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.

There's also a big difference between starting a competing business like your example, and being barred from say working on "cloud infrastructure" because your previous employer also worked on "cloud infrastructure". It can be blurry for executives, but in general noncompetes seem to be used to push pay down more than for any legitimate business purpose.

> Often an acquisition of a company is for the set of customers.

That's a merger. You can, not having any business currently, buy yourself into one. In which case the acquisition is purely for the profits.

> I’ve just undercut what I just sold.

No you've just competed with them. If your prices are lower then you've undercut them. If their prices are artificially high then the market, a.k.a. those customers, are the ones to benefit.

> but I shouldn’t be able to compete directly with the business I just sold

Competition is _competition_. You didn't buy a market you bought an opportunity. You still have to compete against everyone else.

> I just sold using my insider information of that business.

Insider information? On a lawn care business that has no issued securities?