That's not free anymore, but it's technologically easily possible to drastically reduce the amount of resources we use. What holds us back is that it's hard to get other people to do stuff like supporting new file formats or even have a better output in their image manipulation program.
No arguing needed. It's a different format. It uses a different extension ("woff2" instead of "woff"), a different magic number, a different Universal Type Identifier string, and a different @font-face format.
> it's a relatively easy change
But it's not. Even WOFF2 isn't supported everywhere (according to http://caniuse.com/#feat=woff2). And that's a format that's fairly recent (the W3C Recommendation doc for WOFF 1.0 is dated December 2012) and only has a handful of implementations to begin with.
PNG is a format that's a lot older and is decodable by practically everyone. Even if there's only a handful of distinct implementations (and I don't actually know how many implementations there are), and even if every single implementation updated immediately, it would still take an incredibly long time for it to get deployed widely-enough to actually use as a general-purpose format.
It's also worth pointing out that web fonts have built-in fallback behavior (e.g. if a browser can't handle a WOFF2 font, you can provide a WOFF font as a backup), and they're also things that are typically set up once (so generating multiple font formats is reasonable). Images don't really have fallback behavior. On the web, the WHATWG HTML Living Standard defines a <picture> element that provides fallback but it's not supported everywhere. Outside of the web there's typically no way to do fallback either (if your browser can render an image but nothing else can, saving that image to disk isn't very useful, and sending it to someone else isn't very useful either). Also, while font files are created very rarely, images are created very frequently, and most people aren't going to want to create "PNG2" images if they also have to create PNGs and deal with fallback (just look at WebP, which was released 5 years ago and still AFAIK is not used by very many people outside of Google).