It’s because (according to this thinking), a legal background combines well with almost anything else you end up doing as long as terms, contracts, long documents and regulations are unavoidable. Other fields are also useful, and the rigor from studying, e.g., medicine, may be useful for the discipline it instills, but aren’t as ubiquitous and scary to laypersons as law.

It’s why you see a lot of COOs and CEOs with some legal background or at least familiarity.