And having to bring in support for formats that are deprecated by the OS, if they decide to keep supporting that format as there is sufficient demand from their users, is worse than having to bring in support for all formats rather than getting support from the OS?

Having ask that in a slightly confrontational way, one of the reasons I started using VLC all those years ago, and still use it to this day, was having trouble with other media players that relied on OS support fail to work well (or at all) with some codecs, while VLC brought support for them, and their dog, built-in and reliable. Dragging your own format support libraries with you can be beneficial.

This shows the unfortunate power of distribution.

It doesn't matter if the alternative is technically superior once the majority use the mainstream thing.

I'm convinced that this is because of the prevalent MVP culture in modern software engineering. Instead of holistically looking at a new feature request such as "support webp images" we break it down into parts (e.g. "serve webp" "accept webp upload here" "accept webp upload there") and then we call it a MVP when only the highest priority items are done.