I kinda wondered that, too, which is why I went ahead and built this. I already had keycaps and switches, so it was just a matter of getting the case, plate, and PCB, which didn't end up costing all that much, and it proved to be a pretty fun (and ultimately useful) project.
After fiddling around with it, the main advantage I can see is the fact that you don't have to move far from the home row. I'm still breaking my existing muscle memory, but using Fn+WASD as cursor keys and Fn+numbers as function keys is starting to feel fluid and far more intuitive. When you really think about it, with a sane layer setup, 60% really isn't much of a step back from TKL. Just merge the function keys and number row, then put the arrows and navigation keys in an area that makes sense to you. I mostly use macOS now, so print screen, scroll lock, and pause/break are basically useless to me (in macOS, you use Cmd+Shift+3/4 for screengrabs). As long as you don't put your Fn key in a weird spot, nothing is far from reach.
I couldn't imagine jumping into a 60% layout without a flexible firmware, though. One of the reasons I decided to go the DZ60 route instead of ordering yet another GMMK is because it truly gives me the freedom to set this thing up however I want, giving me some wiggle room in terms of button layout (most DZ60-based layouts I've seen use a full-width right shift, for example) and giving me a dizzying amount of control over the way the keyboard actually works. Like, if I wanted to set this thing up so that LShift+RSuper+U+ESC types out ö, I could do that. I don't think it's much of a stretch to consider a compact, fixed layout damn near useless when it comes to keyboards.
I'm not sure I'd want to go smaller than 60%, though if I could get my hands on a cheap 40% PCB I'd be willing to try it. It's easy enough to 3D print a case for one if I just want to derp around with it for a bit. I've also developed a growing interest in split layouts. Not sure how aggressively I'm going to pursue that one, though. I think I spent enough on keyboard stuff this year, considering I'm on my fourth keyboard (shitty Logitech G910, GMMK, GMMK TKL, custom compact), sixth set of switches (Romer-G, Kailh Box Browns, Zealios 67g, more Zealios 67g, Holy Pandas, Everglide Dark Jade), and own four full sets of keycaps. At least I was able to give my old GMMK/Zealios set to my dad, and if it wouldn't make such a mess I would loooooove to shoot the G910. Repeatedly. With .44 Magnum FMJs.