Good question. Three reasons that I can see: (1) cases related to the same parties are assigned automatically to the same judge, since it is presumed that judge will be more familiar with the cases and will not have to spend time informing themselves about the case background; (2) judges are supposed to recuse themselves if a conflict of interest exists, but are trusted to make that determination for themselves; (3) the appeals court system is predicated upon judges honestly reporting the bases for their decisions and fairly considering evidence; if a judge claims to have looked at evidence he in fact did not, the appeals court can do nothing about it.
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