Why are CD's decling in quality?

At some point all CD's, DVD's, LD's, etc will deteriorate.
The reality is, like anything else, they just don't last
forever. But should last a very very long time.

Some environmental factors will speed up their demise
however. And this may be what you are encountering.
Especially anything in sunny rooms and hot/cold temperature
changes. Also just handling them, bending them slightly
(over and over and over through the years) can eventually
damage them or cause the layers to start to come apart
inside I'd imagine (small enough to not be visible to the
eye).

Laser Discs have been deteriorating faster it seems. They
had a real bad rot problem years ago where they went bad
much faster. It might have been a quality control thing I
don't remember but it did just speed up what was going to
happen eventually anyway.

Remember this stuff is just aluminum sandwiched between
plastic. Which is very durable but can and will deteriorate.
That can also depend on how good the quality of
manufacturing and materials were when it was made. These
days everything is so dang cheap. Made in china to save
every cent they can. Nothing is made to last. The skimp and
take shortcuts on everything. I wouldn't doubt you get bad
CD's from time to time right out of the box.

KAcz HEvac, etc

I think it is in the quality of materials and manufacture,
after all in the 80 s the discs were made thicker, the ones I have from then still work fine. It's not my environment, which is a normal home one for them
However, newer ones and it seems to me the newer the worse they are , do skip and as you say they have become like vinyl records. Which were horrible in that regard.
The selling of cds in the 80's focused on people' s frustration with LP's , the scratch and the skipping, were so common and difficult to counter act the effects of, since the CD was supposed to eliminate all that , CD's took off in popularity, that is one reason anyway.
The manufacturers at the time , applied reason to their actions and made the discs of higher quality, That is my opinion anyway based on experience.
Now they have us, Or did till ipods took over, they started making discs poorly , either because they assumed we would just run out and buy a new one, or due to prices dropping for CD's, especially once people could make their own discs at home. Cd prices fell.
this really bugs, me. I order new CD's and then have the great displeasure of having to skip over tracks just to hear part of the album! And, I feel taken advantage off, since they roped me in with CD;s that did not skip and scratch.
Perhaps, it's in the type or quality of the plastic they use. Otherwise , why do Japanese discs work and sound much better at twice the price of our CD's?

Polycarbonate has always been used for the clear layer in CDs so newer CDs should not be more liable to wear and tear than older CDs.

Multiple lasers? Now, That's funny!!

Courtesy Wikipedia:

"Many early LDs were not manufactured properly; sometimes a substandard adhesive was used to sandwich together the two sides of the disc.[citation needed] The adhesive contained impurities that were able to penetrate the lacquer seal layer and chemically attack the metalized reflective aluminium layer, causing it to oxidize and lose its reflective characteristics. This was a problem that was termed "laser rot" among LD enthusiasts, also called "color flash" internally by LaserDisc-pressing plants. Some forms of laser rot could appear as black spots that looked like mold or burned plastic which cause the disc to skip and the movie to exhibit excessive speckling noise. But, for the most part, rotted discs could actually appear perfectly fine to the naked eye.

Later optical standards have been known to suffer similar problems, including a notorious batch of defective CDs manufactured by Philips-DuPont Optical at their Blackburn, Lancashire facility in England during the late 1980s/early 1990s."