The R65 bikes - even the earlier, 1st gen models from late 1978-1980, are still different from the R80/7, R100/7 contemporaries. While many parts of the R80/7 and R100/7 are directly interchangeable (frame parts, swingarms, many engine components), the R65 series has its own unique frame, body parts, seat, and a number of engine parts. Now, various parts are still interchangeable (such as rear wheel, final drive, transmission, starter, bean can, etc.) but quite a few parts do not.
So, it wouldn't make much sense to refer to it as a /7 as that would tend to infer a higher level of interchangeability. There also was an R60/7 model that terminated in North America in either 1977 or early 1978. The R65 models that filled in the gap after that were an almost completely new design, and which served as a test platform for a number of changes that BMW eventually included across the rest of the airhead model line.
Also, 1978 was basically the end of the /7 model designation for the airheads (for Nother America, anyhow). For the 1979 model year, the R100 was called the R100T, and it also became the first year of the R100RT for the fully faired version). The R80 followed this "re-identification" pattern as well.