Jazz artists do this all the time, and it's not a big deal. Maybe the difference is that Jazz often focuses more on the individual, whereas Rock tends to focus more on the band, the ensemble. Yeah, there are exceptions, but I mean in general. If four well-known Jazz guys decide they want to get together and make an album, they just put all four names on the cover and put it out there. If you're a fan of any of them, you'll hear about it and check it out.
If four Rock guys get together, it's almost a given that they'll come up with a band name and promote it that way. And because no one has heard of this band before, the promotion is more about letting people know who is in the band, which leads to lists of what bands they've each been in and who they've played with. I agree with Stadler that to some extent, the word "supergroup" is often overused, but it's gonna happen when the bassist from some band and the singer from another and the drummer who played on a bunch of people's solo albums all get together. The hype is a weird blend of the persons themselves and the bands they've played with.
Asia was my first supergroup disappointment. King Crimson had broken up again, Yes had broken up again, ELP was gone as far as we knew, but suddenly here's this band with Wetton, Howe, Downes, and Palmer. Holy Shit! Blind buy for me. I took it home, put it on, and listened to a bunch of "regular" rock songs no better than anything by any other band out there, and I've tried a few times over the years to get into the album, but it's just boring. No adventure, no fun, no prog.
That was the early 80's, and because of that, I've never bought into the "supergroup" hype. Don't be impressed by who they are and who they've played with. It still comes down to the music. It could be great, or it could suck. I think the fact that some of my favorite guys are in the band might lead me to check them out sooner, but that also leads to disappointment more often than not. If it sounds like old-school stuff I love, people will cry about them being stuck in the past or just cashing in on the old sounds. If it's newer-sounding, people will cry that they're trying too hard to be modern and relevant. The "perfect blend" of old and new that I'd probably find the most interesting is also the most difficult to achieve, and that mix will be different for everybody.