For installs, mysql would take about 25 minutes to compile which is too long (system depending). Also mysql is totally untouched by any customizatins so their binaries work just fine. If you have the time to wait around, then the mysql compile in theory would be fine.
Exim we do binaries because it's a pain to get compiled for some reason. There always seems to be some weird issue that I can't sort out, like the perl.o module for freebsd. I can't get it compiled on some boxes so I use static binaries from other versions which are compatible.
Apache/php etc are all more lightweight and friendly for compiling.
If everything were always able to compile on all OS's I would probably have made everything from source in the first place, but some just have really picky requirements, so we went with the quick-install and no-compiling-pain for some.
Note, back when we first started selling DA, apache and php were rpm based as well. That didn't last very long because of the newer OS versions and people wanted to customize the compiles. Ususally mysql and exim also don't need custom compiles as they don't have has many modules as apache+php do.
John