Because they were created at a time when xft fonts didn't exist. You need to keep in mind that CDE is a project from the early 90s and introducing more modern features would not only require a lot of work, it would also introduce a lot of new dependencies. It's a matter of discussion whether or not it makes sense to introduce XFT.
FreeBSDBuild
It's a hopelessly outdated version of the CDE source though. The current git version compiles perfectly fine on Ubuntu, so I see no reason to revert to such an old version.
The current version is not yet in a state to be shipped. The dtinfo does currently not build and the help system is missing
That looks like the default fallback option in /usr/dt/bin/Xsession (line 171). Did your ./installCDE report any missing files?
SupportedPlatforms
SupportedPlatforms
CDE on Windows 10