The core problem is that setting up and running a crypto exchange is a security nightmare, is extremely time-consuming and is largely thankless (both socially and financially). The upshot of this is that the whole idea is quite unattractive to someone with technical skills and who likes math coins.

AFAICT, Freebitcoins and Freiexchange are the closest realisations of your concept, the former has been introduced upthread in Bayareacoins' own words, the latter was originally created to provide a market for Freicoin, subsequently expanded modestly (in cryptoexchange terms) and continues to concentrate on providing an exchange service to many, if not most, continuing vintage altcoins.