There are a couple of reasons, actually. For one thing, we're already using tapping on characters to bring up zoomed-in versions of them (an important feature on low-resolution Palm displays). Of course it would be even better if you could bring up both a zoomed-in version of a word and its definition at the same time, and that's something we're looking into for the new version, but it'll take a fair amount of work to implement and so we're holding off on it until we're sure that's what we want to do.
Secondly, our font display system on OS5 devices doesn't register character taps properly; OS5 users are already sufficiently irked about the fact that they can't use the zoom feature on their devices that we don't want to add yet another thing they can't access. We'll definitely be fixing this in the new version, but again it's going to take a lot of work and hence we haven't had time to yet.