In order to split the two strands of DNA, the DNA need to be denatured at a temperature high enough to allow the strands to separate (at 95 degrees). I think the reason why they do not use DNA helicase is that the whole process would not occur fast enough (in comparison to simply using heat) to amplify multiple copies of DNA or that they are unable to obtain DNA helicase from bacteria that are able to withstand the high temperatures that occur during the PCR process ( TAQ DNA polymerase is used in the process and is found in bacteria that thrive in hot springs.)...Though my assumptions could be wrong!
so... basically, in PCR, the two strands of DNA have to be split, so therefore high temperature is required, to denature the DNA to allow it to split.
But when compared to DNA replication, the entire two strands are not split, and the strands are just opened up upon the replication fork, so DNA Helicase will suffice. (plz correct me if wrong) and thanks howlingwisdom for your help, greatly appreciated:D