Why no tsuba?
"Because that's not the way we do it in this school."
Beyond that, I would note that a decent tsuba will cost as much as a bokuto and more than a bargain judo-style keiko-gi, and also note that aikido has had an orientation toward keeping entry-level equipment prices low for 60+ years. Materialist explanation often misses part of the story. That's why I started with the oft-heard quotation above.
Hope this helps.
Yah, so many simple questions I had when I first started with sword crafts were answered that way. Only because later when I learned more I found the questions actually made little sense. Funny how that works.
And I love Kim Taylor's stitched leather tsuba for jodo practice. Those things are beasts...