I find it easier to just download the Linux tarball instead of installing the platform and plugins from aptitude.

I did that a few times, but it was a pain. I strongly prefer using the apt repositories since things integrate well.

Probably because Debian still has 3.8 (even in unstable) and they don't maintain/update the package themselves. If you want the latest packages, you might need to look elsewhere. Fedora for example already has 4.4 in rawhide and they might push it to stable in a few days.

My understanding is that 3.8 is a backport of Juno (AKA 4.2) features on the 3.x platform.