1) Playing without external tools is more challenging.
Challenge is not intrinsically desirable a priori. Also, interface challenge is false challenge.
2) Dwarf Therapist usually takes quite some time to update when new versions come out. Without waiting for it, one can start playing the newest version immediately rather than six days and three bug-fixing updates later.
Wow, six whole days! If I didn't have a job, I might notice.
3) Dwarf Therapist looks out-of-place with its white background and non-pixelated fonts and other newfangled thingamabobs. As you have said, DF's interface belongs in a museum, and some like the way it looks.
Dwarf Fortress looks out of place with its needlessly archaic UI components. Personal preference has nothing to do with it: an interface has one job, facilitation. DF's UI does the opposite. Therefore, it is a bad interface. Bad design is not dependent on the observer, and aesthetic appeal is not a goal of interface design (although the UI fails at this as well, so that wouldn't be an excuse anyway).
4) Some people, like me, have attention disorders and are prone to information overload. Seeing a huge table with lots of blue and white and blue-white thingies that detail a crapload of data can be rather frustrating for them.
Actually, tables organize information and make it easier to read. If you have difficulty reading something as simple and basic as a DT table, please sue your public school system.
Also, they may need to constantly trace rows and columns with their fingers in order to make sure they are reading the table correctly, which is pretty annoying all by itself.
Clicking on a heading highlights that row or column, eliminating this problem. This is called good interface design.
5) Some of us have played DF since before Dwarf Therapist has come to light. We have built up a tolerance for the inefficiency of most forts.
This is literally the worst possible argument for not improving the interface's design. The interface continues to be terrible regardless of your feelings about it or ability to cope with its terribleness.
Also, there's no reason we can't have a "legacy ui" option and have it both ways.