There's two primary reasons: 1) Because cards and coins can be made to exacting specifications, but chips cannot (they go through a very manual, human process rather than the machine processes that create and package chips/cards), thus the standards of "mint", "proof", and grading that are applied to cards and coins don't really have a proper equivalent in the chipping world, and 2) Chipping as a hobby doesn't have enough bona fide grading experts to make the slabbed grade actually mean anything.
You asked earlier about dirt and/or cleaning devaluing a chip. A dirty chip in SU condition should be worth an equivalent value as the same chip in SU condition but relatively clean. That said, the cleaner chip can likely fetch a higher value on the open market to the right party. Some collectors don't like vintage chips to be clean unless they are in new/uncirculated condition because they feel it makes the chip look fake. Others want their used chips just as clean as new chips. There's no agreed upon standard here, other than that grime doesn't devalue a chip unless it physically damages the chip (such as if it leaves a stain, erodes the chip, etc).
The link you were provided above contains the same grades, standards, and terminology adopted by the two most widely-referenced price guides, The Chip Rack, and The Official US Casino Chip Price Guide (the latter of which is no longer being updated), and is what is most appropriate to use when describing conditions of single chips. Those are the standards set forth by the Casino Collectibles Association, also known as the
CC>CC, which is an officially recognized association by the American Numismatic Association.
When discussing singles, please try to reference them according to the standards contained in that link. When discussing sets or chips in quantity, you've been guided very wisely to simply look at pictures and ask questions. I would caution you against using the term "mint" in regards to chips, regardless of quantity, as none actually are - all compression clay chips have been handled multiple time by human hands, and largely in dirty conditions.
For more information about the Casino Collectibles Association, please head over to this link:
http://www.ccgtcc.com/