When all the development happens under the same roof in a vertically integrated company, things tend to be smoother and quicker. All teams can work seamlessly together and there should not be information barriers (at least in theory). If you have to work with a partner (or even worse, multiple partners), a lot of time will be spent on evaluating what details can be shared and with whom. In the end, the likes of LSI, Intel and Micron for instance are all competitors in one market or another and giving too detailed information may give the opponent an unwanted advantage. There is also the tradeoff angle: when developing a product for multiple partners, it is impossible to build a product that would meet everyone's needs and wants.