I'd say there are a few aspects that make VLC an easy, common choice: 1) it has incredibly extensive functionality (even if MPV has more capabilities, with VLC, it's all available through menus/GUI - no need to memorize any commands) 2) handles any files you throw at it EVERY time - out of the box. Throughout the years, VLC has been the only player which didn't choke on ANY file I threw at it. 3) It's multiplatform and works (in my experience) equally well on Windows, Linux, and MacOS - OUT OF THE BOX! Even on Android, where I normally prefer using MX Player Pro (bc it has a slightly nicer UI/UX), it was able to handle video files where the audio was in the DTS format. MX Player Pro required the installation of codecs to make the audio work. 4) It's been around forever, so if you want to look up any info related to it, you're sure to find it more easily than for any other player.
All that said, VLC is not always my video player of choice. On Windows, I typically prefer PotPlayer and MPC-BE. On MacOS - IINA. On Android - MX Player Pro. But I always keep it installed on all those platforms because I know that I can fall back to it reliably at a moment's notice and it will simply work.
Additionally, with regard to MPV, where I think it makes the most sense is for the highly discerning user who wants to tweak the rendering of video in a specific way. I think anime fans really like it a lot (I'd imagine they upscale or improve the image quality in very subjective, specific ways, though I didn't investigate this in detail). The other good use case is when you have somewhat limited hardware and you want the most efficient video player application. That is perfectly valid, I suppose, but I think for most people, whatever hardware they're playing a video file on is likely more than capable of running with a more 'unoptimized' video player app such as VLC.