I felt the same about Pink Floyd for a long time. Compared to Gentle Giant or Yes' mid 70s stuff, a lot of their music seems pretty simplistic, compositionally and technically, on the surface.
But then I made up my mind that Prog Rock was more than being a virtuoso and playing 20 minute songs with odd time signatures. It was also about, you know, being "progressive", coming up with new sounds and ideas, and thinking outside the box as far as what the majority of your contemporaries are doing.
That's what the Beatles did with Sgt. Pepper and The White album. That's what Radiohead has done since their 3rd album. And that's what Floyd did on Dark Side to the Moon. You can listen to that album today, and it sounds like it could have just been released. And there is a sophistication to DSotM that takes a bunch of listens to truly appreciate.
(To be fair, 1971's Meddle, the album before DSotM, had a 23 minute long song that was without question, prog).