Why does PCR use heat as opposed to helicase like in semi-conservative replication in order to separate the double DNA strand?
Simply because it is ways more practical and there is no need to use a helicase. Heating is fast and convenient and denaturation is reversible. Also all the DNA is denaturated, so afterwards, primers can bind to their target sites, setting up the start point of the polymerase.
If you want to do this with and helicase, you need to make sure that you have a heat stable polymerase (at least at 72°C), you need to have the right places on the DNA unpackaged and finally, you need two enzymes in the reaction (which is more expensive than just having one).