why does DNA polymerase require an RNA primer?

The lagging strand of DNA is the strand of the DNA double helix in which the template strand is orientated in a 5' to 3' manner during DNA replication. Its complementary daughter strand must be synthesized in a 3'→5' manner, because the overall replication process can only proceed in one direction at a time. Because DNA polymerase cannot synthesize in the 3'→5' direction, the lagging strand is synthesized in short segments known as Okazaki fragments. Along the lagging strand's template, RNA primers are built in short bursts. DNA polymerases are then able to use the free 3'-OH groups on the RNA primers to synthesize new DNA strands that run in the 5'→3' direction.

DNA polymerase can only add nucleotides to an existing strand. It can't initiate a strand.

I suspect this limitation may have to do with a practical size limit. DNA polymerase got the proofreading functionality (which requires detection, exonuclease, and backing up ability -- several subunits).
RNA polymerase didn't get the proofreading, but did get the initiation functionality.

Needs a3'OH on which to add the next incoming nucleotide.