Can someone explain the difference between multiple master fonts and variable fonts? I found MM fonts to be super-nice in the nineties. I could emphasize thing

Can someone explain the difference between multiple master fonts and variable fonts? I found MM fonts to be super-nice in the nineties. I could emphasize things and keep a very nice visual hierarchy while maintaining very nice stylistic consistency. I've been confused as to why we can't have them in 2023. These aren't nearly as pretty as the fonts I used close to three decades ago. Fonts programs are copyrightable (the things which render the bitmap), but font glyphs are not. Even if they were, there are plenty of beautifully-typeset books from hundreds of years ago. Digitizing those should not be hard.