Why does nature rely on RNA primer for the start of DNA Replication? Why not simply use DNA primer and make life simple !
Biochemically none of the DNA-dependent DNA polymerases involved in DNA replication have the ability to begin elongation without a 5' to 3' primer.
The only DNA Polymerase that can catalyze elongation without a 5' to 3' primer is Reverse Transcriptase, however Reverse Transcriptase is an RNA-dependent DNA polymerase, and that accounts for the difference.
And if your next question is, but why, the answer is that that is how the systems evolved. Also DNA Polymerases have a much higher rate of fidelity due to error checking than RNA Polymerases do, and as the RNA primers get removed anyway and replaced with DNA, an error here or there in the primer will not matter.
The RNA-DNA duplex is also less stable than the DNA to DNA duplex, so it makes it easier for FEN1 to remove an RNA Primer than it would a DNA primer, assuming that the mechanism of primer removal and replacement after the complementary leading and lagging strands have been synthesized remained in place.