[fencepost]

It's probably both - finding the holders for a lot of music is impossible, but by using BMI or ASCAP you may be able to shift the responsibility to do so to them in their role as PROs.

Not an area I keep up on, but this is interesting regarding how things worked in 2016 and possible recommended changes. https://www.copyright.gov/policy/pro-licensing.pdf Note that it's about Performing Rights Organizations not about Professionals. Of particular interest is the discussion of 100% licensing vs fractional licensing.