IBM is the biggest, i personally think it is a very good alliance and ANTS settled for the very best.
Companies always have to depend on each other though. If you use your logic then Apple would build their own factory with their name on it instead of using Foxconn's factories.
IBM is a well known and established firm, ANTS is not, so it is only logical and smart for them to try and sell their product through IBM. This saves building up a reputation because IBM already has one.