There are, it just gets added if it was worthy of mentioning.

There used to be individual pages for each Beta build of a version up till sometime back but I guess it got to be too much work. There can be anywhere from nine to fourteen+ Beta builds for a version.

That's a shame! I am an rabid beta user (Thunderbrid (32 bit) and Firefox (64 bit) and would love to know "where we're at?" I am aware that the number of betas per version can go quite high...

the development for new features should already be done at the end of the nightly cycle - beta is more for ironing out the last bugs or fixing regressions that just get discovered once the new code hits the larger audience on beta, so there wouldn't be too much exciting stuff going on in beta changelogs.

https://hg.mozilla.org/releases/mozilla-beta/shortlog might contain some information you're looking for in a raw format...