Because they hate standards and they go out of their way to destroy existing standards in order to gain market share with their proprietary devices. The consumer always loses when they start another format war. Once or twice in their history, they have had a better product, and the war made sense, but usually they just start a war and the consumers are caught in the middle. I blame them for the drawer full of compact flash digital camera memory cards that I no longer use. My old camera, my new camera, my old cards, and my new cards are all non-Sony devices, but the war they started with their stupid MemoryStick destroyed the memory card standard that existed up until then.
I wrote a really long rambling post and decided to delete it, but their wikipedia page has a
big section on all the format wars they have started in order to destroy existing standards to gain market share with their proprietary formats.