Oracle can not find out which transactions are commited and which are not from the archive log he is currently applying. The transaction might start in the current log, but it might actually be commited in the next archive log (or in any archive log that is far ahead of the current one). So how would recovery process know which transaction to start recovering and which not?

That is why after the roll-forward phase is completed Oracle has to rollback any uncommited transaction. And it finds uncommited transactions from rollback segments, not from archived or online redo logs.