Why do computer manufactures prefer to build computers from separate standard components?

Cost, cost and cost.

When you design a computer it is all about performance, and to sell it, the price need to be competive. If it is better than competitive, and you can make a device with more bells and whistles, better performance, better battery life, bigger disks and still to a lower price, the difference is the profit you can make. Since standard components are produced in large numbers, these are cheaper than those you design yourself, and make just those few you need. So standard components are preferred, until you elect to make something your competitive edge. This you spend your effort at making better and leave all the other things to standards.