Mobipocket dictionaries are particularly formatted
as dictionaries for use by software. Epub does not have such a dictionary specification in the standard (as I understand it, though I believe, as mentioned, it is being looked at), such that publishers can bring out duly indexed dictionaries for software developers to write reader software/apps that can lookup words from books being read, in those dictionaries.
It is precisely to avoid people like Sony having to write their own proprietary means of enabling dictionary support and proprietary dictionaries (in that, as you specify, we have "no idea" how they are installed or what they actually are), and to enable the simple purchase/install of assorted or specific dictionaries as desired by the customer, that the mobipocket dictionary format is popular with some, and why we see it as something advantageous to be provided under epub (and thus why it is being discussed by those involved in developing the epub standard).
It is, for those of us who desire it, important to avoid epub dictionary-lookup being fragmented into proprietary, non-selectable solutions. I suspect this is a possibility as to why not many epub dictionaries are found, compared to mobipocket - that the dictionary publishers are perhaps waiting for the possibility of a similar, standard, dictionary-lookup epub format to produce and sell their dictionaries in (varying perhaps only for the store's preferred DRM
).
Cheers,
Marc