"Cloud" is just market speak for hosted service.
Well, no it's not. It also means using white-box commodity servers to serve a large software application. The savings from using commodity servers is put back into the software development to make it more robust to handle the less reliable commodity servers.
If you're large enough, you develop the software yourself; if you're even larger, you design the commodity hardware yourself, which allows you to drive out cost while increasing performance in the things you get a return on. Neither of which either Dell or HP can add any value to, so there's just no reason to use them.
Google is the 5th largest server manufacturer in the world by itself. Add in the other big cloud players: Microsoft, Amazon, Facebook and a significant portion of server purchases are going to commodity hardware, whereas 10 years ago it was OEM. And it's not going to get any better. The fact is, building your own white box makes sense for more and more installations, because it's really not that hard. If you need more than about 10K cores, you can probably find it cost effective to start doing it now, and if you are any kind of software company, you already have much of the software development resources in house.