I have a guess about this, somebody please tell me if I'm right
TiMidity (or any other similar program) is not set up as default because there is no complete open source soundfont out there.
And why bother with making an open source soundfont when there are much more relevant issues to be fixed?
Is this the problem? Cause if it is, I might be interested in trying to improve the soundfonts (though not immediately and having no previous experience).