In addition to what others have said, many institutions have a "residency" requirement for the PhD, requiring at least one year of full-time on-campus study. Personally (and I am a department chair), I think that this all needs to be modernized, but I don't expect that to happen for several years yet, even though COVID. There are some online programs, but in terms of admissions you're competing against people who are happy to be on-campus, so whether you like it or not, the admissions committees are going to prefer those people over you.