"Bridge Over Trouble Water" sounds artificial

On the other hand, most of Paul Simon's solo releases sound quite nice on CD.  

I had the same reaction--system got better and S&G got worse.  The same is true for some Who albums.  I was expecting access to the natural voices of S&G but they were processed and the instrumentation seemed disconnected from the vocal tracks. 

The solo work of Simon and Garfunkel have better sonics, of what I've heard.

A good anti-dote is the new Sarah Jarosz --World on the  Ground.  The sound you wish S&G engineers delivered.

...'course, there Is the fact that a few decades have transpired since the originals and all the various remasters, compilations, etc.

Studio has improved, your equipment ditto.  Meanwhile, your 'on board listening system' has likely deteriorated a bit....if you'd care to be honest about it....

Mine has....it kinda comes with the 'territory'....a.k.a age. ;)

I have "Graceland" and "Still Crazy after all these years" on vinyl and I enjoy them a great deal.
Last Winter, after listening to them, I got nostalgic, thinking I’d like some of the older songs that I grew up with. On eBay, I found a NM copy of Simon & Garfunkel’s "greatest hits"...released in the mid 70’s. OMG, it was the perfect example of "one and done". The sound took me back alright... back to the sound of a battery operated, early transistor, AM radio. I listened to the whole thing, hoping to find one track that sounded good, and gave up. Haven’t bothered to listen to it again.
That was disappointing!

I have original vinyl recordings of S&G. Bridge Over Troubled Waters is an excellent recording and mix, as it was originally mastered by Columbia.

No idea what the OP is listening to, what mix or who mastered it.