In every era of music, a genre, IMO, is more or less defined as “a bunch of lesser lights attempting to recreate the magic of a certain aspect of the work of the leading lights”.
Mozart is not usually pigeonholed as a Classical composer, but everyone for several decades afterward is. Beethoven is not always classified as a Romantic, but everyone after him is.
The same with Prog-rock. It was brought about by several factors: multi-track recording techniques that lent themselves to more complex arrangements; the rise of the LP, which lent itself to longer composistions; mind-altering drugs that lent themselves to ideas of expanded possibilities.
On August 5, 1966, The Beatles released the single “Eleanor Rigby”: Paul sang the lyrics, and not another sound on the record was performed by a Beatle. Producer George Martin thought a string arrangement was good for the song. The Beatles were popular enough at that point that they didn’t need to worry that anyone might regard this as a sell-out.
On October 28 of that year, the Kinks released the Album “Face to Face”, which included some fo the first recorded tracks using a Mellotron, an instrument that combined the sound of an orchestra with the convenience of a keyboard.
On December 1, The Who released the album “A Quick One”, whose title track was very long, with multiple differing sections (the thinking being “Hey! If those Beatles bastards can do a string quartet, we ought to be able to do an opera and get away with it!”)
All of these bands continued on from there, trying out different things, but it was the bands that were inspired by these actions, and who recycled them into oblivion for the rest of their careers, who form the core of prog-rock.
Progressive rock can be summed up as lengthy songs, even suites, combinations of rock and orchestral sounds, and chord progressions far more ornate than your average 3 minutes of girls and cars that spelled ROCK prior to that.