Because of money. Times when a usual desktop printer has a better embedded computer than the general-purpose desktop computer driving it are long gone and the process of actually getting the data in format which will be displayed by a computer to a format suitable for printing is quite complex. Also as time passes, expectations of users about the complexity of image to be printed and quality of the print itself increase.
There is need for corporate secrecy because today one of the most important parts of a modern printer is how it actually processes data on the host computer. Just like we had hardware fax modems and then switched to software ones which are cheaper, today we're in age of "software" printers (some call them "winprinters" because they usually only have drivers for Windows) that do all of the data processing for the printer on the host computer and then send data in a proprietary and closed format to the printer. The printer will have hardware optimized for that particular format and since printers are made in large volumes, the custom hardware will be cheap. The embedded computer in printer will then convert the data sent to it by the personal computer to something it can actually print. Since, as I said, we already have optimized formats and hardware (and firmware) which is made to work nicely with them, the hardware itself can be cheaper and can quickly process the data that needs to be printed. This way for example a printer which has only say 16 MiB of RAM can work with large photos that themselves are much larger than its own RAM.
Companies themselves make money by selling printer supplies and by licensing the printer components. This way, they benefit from secrecy because there would be no reason license use of hardware if all implementation data is free.
Also the idea you proposed here is quite reasonable and back in the days of expensive printers, we've had printers that can directly print pages in PostScript and PCL. Such printers didn't need any special drivers at all. Today such printers are rare and are mostly big stationary units size of pieces of furniture. So while currently used printing technologies usually aren't very new (usually computerized xerography or inkjet), the technologies for cheap low quantity printing are developing quickly and are new.
UPDATE I did notice that that some manufacturers like for example Brother or OKI do have relatively cheap printers that do support common page description languages and may not need some special proprietary drivers.