Well, most of us don't know what the "original" sound was. We're just trying to reproduce what we think it should sound like.
Also, I'm guessing that all studios aren't built the same and don't sound the same. I'm pretty sure many of them use different amps and speakers for playback. Like you said, some use tubes, some solid state.
So when you say "why change it"? I don't think the question, or more specifically the answer, is that easy.
I think most of us feel our home systems are some sort of compromise. I've got about $10k US in my system. Most studios are considerably more. My system is in my family room, which I don't really mind, but it would probably sound better in a dedicated room, or better yet a specially built room. I've got other furniture in the room and a sliding glass door. Compromises.
So the product they created is going to sound different in my room, on my equipment, than it does at your house. How do we know which one is more correct? Unless you were involved in the recording and heard it in the studio... we don't know.
Why do some songs sound bad regardless of what they are played on? I'm thinking of Go Now by the Moody Blues, for example. Didn't they listen to playback in the studio? They thought it sounded good? Good enough? Did it actually sound good in that studio?? No one thought to say "Hey, the vocals on this recording are overmodulated and it sounds like shite"?
And like the other poster said, personal preference plays into it as well. I don't think most people are actively trying to 'change' the sound of their recordings, just trying to make them sound as good as possible. And I don't mean one song by one artist; I mean all (well, most) songs by all the artists of different genres that I listen to. Rock, acoustic, country, progressive, old and new etc. It might be easier if I only listened to one type of music but that's not the case.