I believe that there are a lot of variables at play here.

Many of these are/were created by the casinos for a variety of reasons. Commemorating a Grand Opening, or new building, or restaurant, or just about anything, and given out to players, guests, executives, employees or who knows how. Encasement in acrylic resin does in fact, damage clay-based chips, in a chemical reaction. Just look across the surface of an encased chip with a magnifying glass and you will see deterioration of the surface and the chip is fused with the acrylic. A clay chip cannot be effectively removed and will always be "damaged" by this process. (Note: metal tokens can be removed. I am unaware of trying to remove Bud Jones plastic chips, ceramic chips and others, but suspect plaques and jetons are also damaged by the process.)

There are several instances where a given chip does not exist outside of an acrylic encasement, yet, the chip is listed in TheChipRack, with a value code. I do not see encasement to be akin to slabbing in any way. Slabbing allows removal, encasement does not. (Note, I am against slabbing.)

I am not certain who, but over the years many, many low value chips have been cast into resin squares and are sold at retail venues. Spinettis Gaming Supplies comes to mind, and I have purchased many from there to add to my collection.

Sometimes, these cast resin paperweights can be spectacular in their look and can be quite expensive.

Here are two from my collection...

For several years, this was my representative MGM Bicentennial chip in my Bicentennial Collection

This crystal ball casting also has the $5 Main Street Station chip. I have one of those in my collection, but not a $25.