It's not the app. It's the system. The dictionary is system wide and the settings are the same. That's because the iPad really doesn't have room for every app to load and use it's own dictionary. At least Apple isn't going to add them individually to each of it's apps.
Full blown, mature, computer programs are always going to have more features than tablet apps. Both because of the space available and because most of them have been around long enough to go through several generations of feature upgrades.
That said, yes, Apple should add some dictionary editing abilities to the system dictionary.
Please Apple, if you are listening.