This is a super simplistic version, and doesn't do justice to the work involved.
It's the nature of software development. We work in branches, so while we have our launch day branch any spin-offs (betas, demo, E3, gamescom etc) all get created from it.
The beta was taken a few months ago from our main build, and from there you then go into hardening mode. This is where you focus on bugs, stability and no further work should get checked in. With each check-in, comes the risk of new issues coming up (as is the case with Software Development). We're making so many changes to the build so you have to make a call between locking the build or stopping active development.
We don't want to stop development of our launch build, so you have to 'cap' the branched build (in this case the beta) at a specific time.
This won't be unique to us, this is how the industry will operate as a whole.
The same thing will happen to release builds the clo...