getting this out of the way:
https://suckless.org/rocks/
http://harmful.cat-v.org/software/
https://github.com/mayfrost/guides/blob/master/ALTERNATIVES.md
okay, to actually get to what you're asking, OP, first you have to define what the fuck you mean by a modern browser; a lot of software projects become bloated over time due to feature creep. If by a modern browser that implements the entire paintrain that is some sort of Javascript and plugin support, then how does it use resources on the target machine to achieve this goal?
>Why hasn't it been done when it seems like more than enough would be interested.
That's just it
for real RAM nazis we already have w3c, links, lynx, etc.
for "just good enough" people there's already things like Dillo or Surf
if you want to create a new project for a fully-featured browser then you are competing with all of the other forks for people to actually support with code contributions, which is to say unless this project was the work of one Terry-tier madman it needs to create a public image of something worth contributing to and supporting, and this would take a clarity of vision to define exactly 1. What does the browser do and do well? 2. how does the browser do it? 3. what does the browser not do? 4. what software implementations does the browser use to accomplish this?
If you can answer these in a concise and positive way, and these conditions aren't overlapping with that of another browser, then you have a chance.