Traditionally, externships were unsupervised internships. The traditional quality of externships has been extremely low. Even when externs have volunteered to work for free, few law firms have been willing to accept them. This is inappropriate for law unless you want to require a year or two experience as a paralegal a requirement to go to law school. Some types of programs require just that sort of experience.
Law schools can provide supervised clinical internship experience.
This is the norm in education. In every professional program except for law and business, the schools provide and coordinate supervised clinical internship experience both before and after graduation. There are excellent reasons for this. First, supervised clinical internships are a part of the educational process and so are provided by educational institutions. Those who create and profit from a problem should be responsible for solving the problem as well.