In some cases we have audio but it has not yet been processed. It takes time to go through old recordings, digitize them if they are on tape, and work through them and cut out individual words.
In some cases, the word was recorded when audio recording was not readily available, or the word was heard when not recording, e.g. in conversation.
Some words are difficult to obtain in isolation. Some words are always used together with other words and speakers may find it difficult to bring them to consciousness as separate words and speak them separately. In some cases, it is difficult to tell a speaker exactly what word we want and can only elicit the word by asking for a sentence that contains it.
Although it might seem possible to cut words out of recordings of sentences containing them, words are not generally separated by silences and attempts to cut them out of their sentential context result in distortion.
Sometimes it isn't clear what root a verb is based on.
Yes. Only two aspects of this dictionary depend on a network connection to other web sites: (a) the maps in entries for place names; (b) the external links, which are mostly to information about living things. It is possible to install a copy of the dictionary on your own computer or cell phone, any device that has a web server, and view it in your browser.