I've heard and attended a few ketivat ot ceremonies for the writing of a Torah. I've never heard of a similar ceremony for tefillin or mezuzot. I assume that it may be that these items have far fewer letters? Nonetheless, there certainly are enough to write to have a few people assist in this. Is there such a thing as ketivat ot for these items? If not, how come?
There is a distinct Mitzvah to write a Sefer Torah (specifically, mitzvah number 613). This can be accomplished by filling in a letter (as the Rambam in Mishneh Torah, Tefillin, Mezuzah and the Torah Scroll 7:1 writes explicitly that even writing one lette fulfils the requirement). Since most people aren't writing their own Sifrei Torah, there are these 'events' that allow people to join in with minimal monetary contribution. There is no Mitzvah to write Tefillin or Mezuzah, therefore there are no 'events' to do so.