Who knows. They may have paid for the music rights, they could be granted certain rights if they're in a network, they could simply be pushing their luck and using it without permission or they could be using it by any other of countless ways.
With music and copyrights, it's always best to worry about yourself and not what someone else is doing.
That all depends on the music, where you found it, and what the musician and songwriters who created it require.
Yes, it does require a lot of effort, but putting in the effort is better than having your video removed and getting a copyright strike or it becoming 3rd party claimed so you can't earn money on it.
It also doesn't help that there is a slew of bad information being posted and people using terms interchangeably when they mean different things legally:
Public Domain (not possible in some countries): It has no restrictions, no copyright claim and can be used however you want without risk. Music in public domain is extremely rare.
Creative Commons: work may be used, but only in compliance with the stated restrictions (typically credits or where/how you can use it).
Royalty Free: you BUY a license once and can use the work (without having to pay again) according to the license.
