Generic drug names don’t describe chemical structure, they allude to purpose but that’s all. ‘-afil’ is used to apply to a particular class of drugs, although when it was discovered, ‘sildenafil’ was the only example of that class, so it didn’t mean anything already.

This is like having the first tool of a particular type come along and call itself ‘Mosaic’ and then someone makes another tool of the same kind and calls it ‘Mozilla’.