Because it's lightweight, fast, efficient and still does the relatively static requirement that it was designed for?..."So, why would you still use Winamp?"
That's been my problem with the usual dorks who say "But I just don't understand why anyone would own a DVD / Blu-Ray player in 2020 when you've got Netflix". I do have Netflix and Amazon Prime as well and both combined have barely 5-10% of the catalogue size that DVD + Blu-Ray do. So if your taste in movies is somewhat more varied than Michael Bay, Marvell Super Heroes or the narrow slice of what gets offered in the usual 5-10 year "gap" period that fits in between lacking the very latest movies and simultaneously lacking most classics older than a decade, streaming may be a complimentary service it but it ain't even close to being a "replacement" technology for being able to own from a 10-20x larger choice, even ignoring the way half the catalogue disappears on / off every 2 years or so for "rights" reasons (which certainly doesn't happen with my collection of CD's, video games or books...)"But even with 40 million songs on tap, there’s a significant gap between what I want to listen to and what is available on streaming at any given time."
Since you can play any video or audio file with VLC, installing something extra is no longer necessary.