Why is it not possible, except in rare cases, to appeal a lawsuit before the lower court issues a finding?

The reason for an appeal is to review the judgment of the lower court, so the appeal cannot be made until the judgment is issued.
The reason for an appeal is to decide and determine the level of harm committed by the original trial, so there is a time limit on filing for an appeal.
The reason for an appeal is to test the legal principles at play in the original trial, so the appellate judges must know what those legal issues are.
The reason for an appeal is to review unfair decisions, so the appeal can only address issues of law not issues of fact.