Why not use The GIMP? Three reasons:
1 The UI. The GIMP was written by programmers who had very little graphics experience, and it shows. Using it can be actively painful, as certain users have been known to go looking to hurt someone because of UI choices. Things have improved, but there are still problems, many of which have been around for over 15 years, and I really don't think will ever be addressed.
2 The plug-ins. A lot of users use several of the hordes of Photoshop plug-ins available. Some (not all, or even most) can be used in the GIMP, but usually with extra effort, sometimes considerable additional effort. Certain users might use Obscure Plug-In X once a year... but when they need it, they need it now, and there's no time to wrestle with the GIMP. Sometimes, using a plug-in is simply ensuring that WINE or similar is turned on. In other cases, things are not so easy. (Yes, there are problems with plug-ins and the GIMP on Windows. Yes, really.) In many instances what seems to me to be the obvious way to make the things work doesn't. There might be a non-obvious (to me) way that I might or might not stumble across later. See the UI problems noted above.
3 The weird bugs. The first version of the GIMP that I ever tried could not print on certain printers, including very popular machines from Epson and HP, back in the days when HP was good. If the GIMP won't print with the most common series of desktop grayscale laser printers in the world and with one of the four most common desktop inkjets in the world, that's not user error. Yes, this particular bug has been addressed. How it escaped into public is unknown to me, and I don't bloody care why. And there are other bugs that have not been addressed. Up to the last time that I tried, maybe a year ago, I could make the GIMP on Mac (but not Linux or Windows) crash any time that I wanted to by a simple sequence of steps, a problem present for at least ten years. At this point, I suspect that it will never be fixed.
Affinity Photo doesn't make users want to inflict grievous bodily harm. Affinity Photo uses Photoshop plug-ins so easily that Adobe has taken active measures to move stuff around to make it interesting to use Affinity. Bugs are (usually) fixed quickly. Yeah, Affinity costs money. I'll pay to avoid the hassle. And it's not very much, especially when compared to Adobe's grand theft.