>So how come we don't have have ambigus instead of buffets?

the short answer is that ambigu went obsolete (and didn't mean exactly the same thing) before buffet table/meal/supper/party was shortened to buffet meaning the refreshments themselves rather than where they were laid out.

ambigu - An entertainment at which the viands and dessert are served together; or at which a medley of dishes are set on.

and with regard to transmogrify, OED says "origin uncertain", but has this [Note. If the original form was (as suggested in quot. 1725) transmigrafy, this may have been a vulgar or uneducated formation in -fy from transmigure, or transmigrate v. (cf. transmigration 3b). Apparently, it was originally persons that were ‘transmografied’, or metamorphosed.]