Why interviewers ask when is my contract ending with current company?

Whenever I tell them that i am looking for full time job instead of contract work, interviewers always ask me when is my contract supposed to end. As far as I know, and in my case as well, most IT contracts are open ended meaning contractor can quit any time with two weeks notice or the company can let go the contractor anytime they feel like.

Why do you think interviews ask this question?

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