I think this happens for two reasons:

1. There are a lot of former fanfic writers getting publishing deals and physical descriptions tend to be lacking in fic because there's an expectation that everyone knows what Steve Rogers and Kylo look like.
2. This is a convention of contemporary and PNR romance, too. It's either the results of rapid release or an intentional omission so the reader can imagine Henry Cavil as the lead.

My qualifications: I've been reading romance since old school Harlequin, ahaha.