The issue is twofold: outdated copyright laws and a lack of interest from the current market.
I think most gamers would pay say, Sega a flat monthly sub for a bottomless feast of retro classics they can gorge on but Sega recently shut down their whole retro handing operations!
This leaves us with the legal grey area of abandonware. In my eyes piracy is only theft if you are circumventing financial recompense for the original creators. Downloading an MP3 instead of paying for it? Not cool. Amazon hackbox with Kodi? Definitely not right. Installing ROMs on a hacked Switch you can happily buy from the eShop? Bad.
But playing a ROM of a title long since abandoned by its creators or that you have paid for on an older discontinued digital platform (say the 3DS eshop or PSP Store) that has since been taken down? Fair game I say and copyright law needs rewriting to reflect the fast moving world of software.