A lot of repair options are POSSIBLE, but some turn out to be not PRACTICAL.
Most experienced electronics repair shops would still purchase the more expensive full assembly: cover glass, digitizer and lcd in one. Separating the old laminated parts is too time consuming and risky for a single job. Glass-only repairs only make sense when refurbishing thousands of screens at a time, so that you can afford to by the expensive tools required to do it right.
The factory vendor posting on eBay or AliExpress does not care if their glass-only part is a practical option for their customer. They just want to sell as much glass as possible.
Think of the windshield on a car. This is made of multiple layers of glass and plastic all glued together. If you cracked the top layer of the windshield only, you would never consider stripping it down to preserve the bottom layer by replacing only the top. Anyone would choose to simply buy a factory made assembly of the whole windshield.