The sentences on the site are written and checked by the native speakers on the team.
I don’t think you are wrong that there are some sentences on the site that are not things you would likely use in real life once you get to the level of speaking normally. These sentences are most likely to be found in N5 and parts of N4.
I think the point @lunchbox1 made is pretty accurate. If I wanted to help a Japanese speaker understand how to use “This” and they had limited understanding of English, the sentence “This is a textbook.” is a great way to do it, but I honestly don’t think I have said that phrase out loud outside of a classroom ever in my life. It isn’t grammatically incorrect, but the context in which it isn’t awkward to say that sentence out loud is pretty limited.
I think the same can be said about a ton of sentences from almost any resource, especially for sentences targeted at lower level learners. They are good for helping you understand how the building blocks fit together, but people don’t usually talk in short simple sentences in real life.
That being said, if there are any you feel are really awkward, feel free to let us know!
I will point out that what you feel is odd, or even what any given Japanese speaker feels is odd, might feel totally normal to other Japanese speakers. It really comes down to each person.
