Why didn't the FBI just get a survellance warrant for Trump himslef?

If the Steele dossier was the sole or major justification for a FISA warrant and the FBI was so rabidly anti-Trump and the FISA court was so dim as to not question sources it seems they could have had a warrant for Trump. Why did they go after someone who was not even part of the campaign at that time?
Update: "himself" Bad typing.
Update 2: Here is the missing "i". and mea culpa on the transposed letters. This is to cutoff the nitpicking that some will use to avoid answering the question.
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