It is not the single witness who is saying that the object belonged to Reuven; it is the Chazakah that tells us that what Reuven is holding belongs to him. As long as we accept the testimony of the single witness we deduce that point ourselves.

Until the grabber contradicts the testimony of the witness with a Shevu'ah, his testimony is accepted just as that of two witnesses. If two witnesses had said that the object was grabbed, the grabber would not be believed to say that it is actually his since Reuven was the Muchzak. So, too, the grabber is not believed that it was his if a single witness says that he grabbed it (and that is what he is ostensibly saying in the Shevu'ah you suggest). Therefore the only way to negate the testimony of the witness is by swearing that he did not grab the object altogether, which he as unable to do in this case.

D. Zupnik

(We are assuming that your question is on the Rashbam's understanding of the Gemara -- see Insights. According to the other explanations, the answer would be obvious.)