Why do manufacturers withhold data sheets and manuals?

It seems like manufacturers should want to sell as many chips as possible, and it seems like they could sell more chips if they released the documentation to allow anyone to implement their product. But many devices or peripherals (i.e., ARM Mali, Allwinner CedarX, etc.) are undocumented and need to be reverse engineered. I noticed this when when I was looking at the RK3399 data sheet and saw all of the features this chip has. It's a shame that of all those features, the only ones that will be used on most SBCs are the ARM cores and the simple I/O peripherals (SPI, I2C, GPIO). I don't have the skills yet to write an OS driver, but I know how to write bare-metal embedded drivers using the information in a data sheet, and I wonder why the manufacturers don't release that information for many of their SoC products. What does a company like Allwinner or Rockchip gain by making it more difficult to implement their products?