Shortly after the NES Classic was released, people found out it was easily hacked, and released tools that allowed you to expand the 30 bundled games to over 300 unlicensed ROMs. It all fit neatly into their UI and everything (from what I saw.) I've heard it surmised many times on the Internet (and that makes it true) that they weren't interested in shipping their own 'Kodi Box' equivalent.
However... there's also the stories of 'how to build your own NES classic' using a Raspberry PI or equivalent, including adding all the ROMS you want. Supposedly it's cheaper than the NES Classic as well. The only thing you don't get is the nostalgic (tiny) little box and the cute ( short tailed) controllers.
You could ask, 'why don't they decide to own that market, and just write off the 30 vs 300 as irrelevant?' (They'd already picked the best 30.) At least then they'd get their mark-up. (Whatever that was.)