Because there is no reason to hide the fact that she is married, if she was already so. It isn't surprising she don't even have a bf, much less husband since her current career is so packed.
Concert+Practice+Recording+Seiyuu+Radio+Interview etc, where got time to do other things. =_=
Anyway 28 still considered young, since many other big names seiyuu are also still single, and older than her too.
If anything, there were some really small & lame rumors that she is with Takahiro Sakurai. Very unlikely though, as those are just baseless stuff.