This depends on the company but my understanding is that in most tech companies, there's not much distinction between being an ML engineer and being a research scientist/engineer with a Ph.D, in terms of the type of work or compensation, provided that you aren't in a pure research role (i.e. you were hired because your research is highly regarded by academic peers in a narrow field and get to do something close to original research with fellow academics). Most Ph.Ds aren't qualified for the latter type of positions either and the number of non Ph.Ds that are qualified would be entirely negligible.