Re: "a bunch of really catchy songs stitched together to me"
That's actually why I think Yes's epics stand out so much in the genre. Most (not all) of their epics coherently flow from beginning to end as a fully realised song without relying on being a medley of shorter tracks. "Close To The Edge" for example is formally divided into sections, but you can't pull those sections apart and make them work as individual songs. the whole song makes use of the same melodic material and is structured in a way that makes it feel as if the length was necessary to fully express the musical ideas contained within the song. The thrust of the song comes from the transformation and development of the song's musical ideas rather than running through a bunch of unrelated musical ideas in a row.
Most bands are capable of this, but they usually do it better in the context of shorter epics than on side-length tracks. about 7-13 minutes seems to be the sweet spot for most prog bands.