I bought Spore a long time ago here on GOG and I've played on Windows since then, but I always knew there was a Mac version that had been released. I was wondering why GOG only offers one of them...?

It might be due to MacOS Catalina dropping support for 32-bit, as has been the case for other games.

Mac and Linux versions of bigger games are often developed by a different company, which often retains some rights to the ports they produce. Besides the fact that that would mean extra time spent in negotiations to get those ports here, some of these companies (I can't remember any names right off the top of my head) seem to be really set against DRM-free distribution for whatever reason.

EDIT: I don't know anything about the Spore Collection's case; it might be some other reason.

Even so, I have Mac OS Sierra, so I could still play it, and I'm sure many others can.

I just checked the Wayback Machine, and GOG doesn't seem to have ever carried the macOS version of this game.

There never was a Mac version of Spore. It was always the Windows version running in Wine. (Actually I think it was Cider, which is a slightly tweaked version of Wine.) So just run the Windows version using Wineskin or something, it's the same as the "Mac" version.

I think you are right. The Mac OS version of this game is only for sale on Origin, as Steam doesn't have it either. Be it the normal game or Spore Collection, it seems that EA holds the exclusive rights to the Mac OS version.