Music is a creative work and people may legitimately want to consume it independently. Cast lists (and data in general) don't have copyright protection and serves little purpose except to augment the original file - both the means and the motivation aren't there.
Even if cast lists don't have copyright, they still need to source the data from somewhere. If I was responsible for this at Netflix, I would prefer to buy that data from a good quality source instead of scraping the data from external websites or running OCR on the credits screen.
Netflix adds on average about 1 show per day. It would be incredibly cheap to have an intern just watch the credits and write down the names. Maybe you get a second intern to doublecheck their work. Sure buying access to an already compiled database would be preferable to making your own, but even that's pretty cheap. For IMDB a million api requests per day costs $45/month - if we assume 3600 shows, 5 seasons per show, 10 episodes per season, 300 credits per episode that works out to less than 2 months of api requests, or about $90 total (substantially less assuming you can retrieve multiple credits with a single api request) to create a database for their entire catalogue. I've got more than that in my wallet right now. Of course you probably need an engineer to write a script to make all those API requests and at that scale it will take a little time to write and some effort to maintain, less facetiously it's probably going to be comparable to the cost of a few interns. Still, this is definitely not an insurmountable barrier for a multibillion dollar company.
I'm not familiar with how licensing works at that level, but I know there are a lot of licensing deals involved regarding music in the series and subtitles for example. It wouldn't surprise me if also meta data such as the movie cover and credits may need to be licensed separately for some markets.