It's simply a word-using habit. People like to omit it to make the word simplier.
As someone learning this language for the first time, the inconsistencies make things a little confusing. That's not the only time words or modifiers are omitted in the early lessons and it gets kind of confusing, especially in the multiple choice answers. There are one or two where we choose "all" the best answers, but some of them have words we've never seen, and that are dropped in other uses of the sentence. :/ I know languages aren't consistent in real life, but it does make learning more confusing!