Re: Why?
> I guess all those apps I have installed, not from the App Store, must have been compiled on my machine then...
> They're all signed (an ad hoc signature is fine). This is not necessarily a bad thing, but some people are concerned that over the long run Apple will seek to "iOS-ize" macOS and have only known signatories, certified by Apple.
Its worse than that. Those apps, not from the App Store have been signed by a certificate that the authors have had to beg Apple for (and pay for the privilege). This is the same as Microsoft's Windows RT developer license.
As I mentioned, try compiling a binary on your machine and try running it on your second machine. I bet you can't run it without also begging Apple (and paying) for the privilege. Basically the developers who write software you enjoy using are being forced through stupid hoops by Apple.