People speak weird all the time, and LLMs are trained on people. Some aren't native speakers, some just like to omit verbs, nouns, or tenses when it seems obvious and they want to be expedient, some just do it for fun or laziness (see, l33t speak and or early texting, typos).

LLMs are trained on human input, so of course it on occasion uses our bad habits. Thinking like your comment suggests is what gets people who really wrote their own stuff in trouble, because people think they can identify stuff like this more than they actually can.