The problem is that when it comes time to relicense, there's probably very little value in the show left to the studio. And whoever is negotiating the relicense couldn't care less about the artistic vision of the 10 year old show's creator who picked the original songs. And whoever the right holder of the music is probably doesn't want to offer steep discounts because it weakens their negotiating leverage on other matters.
The main issue is that it's very rare for someone to "own" a song. The publishing, label, composition, mechanical and performance rights might all belong to different entities in different territories and sometimes even in different windows. Relicensing is pretty common for films, but less common for TV, because there's less margin. And collective licensing for broadcast happens in lots of territories and not for on-demand or internet distribution.
Licensing is generally complicated but it's often for some fixed period of time. I expect part of the problem is in the incentives. I get the credit if I can negotiate a lower licensing cost today. If the license expires in 10 years rather than 50, that's probably not my problem.
