Vertically integrated monopolies are inefficient because practically, what you really have in a vertically integrated monopoly is a bunch of separate companies, all following a single, centralized entity and paying tribute to that entity. There is an opportunity for profit here, for shareholders, but very little room to actually improve technology, provide utility to users. Every product in that monopoly becomes less of a computer, more of a mouse-trap to keep people in the ecosystem. If your monopoly has even a single component which can interoperate with other monopolies, then you're basically subsidizing a whole layer of infrastructure.

Plus this practice paints a huge target on your back for anti-trust action, as you are seeing now with the other monopolies. The bigger your monopoly gets, the more potential profit there is for whatever government agent can bust you up. This could result in promotion, christmas bonus, maybe even a corner office with a window. Very high stakes.