Desktops are modular because the high price components used to have. Look at manufacturers now and you'll see a trend towards non-modular designs (iMac, Vaios,…)

This is not a scheme to take your money, BTW. Most of the consumers wont upgrade any part of the PC during their lifetime, so you are being held back in design and build quality by a feature only a fraction os the population demands. By held back I don't just mean design wise, although is one of the main advantages. You have to support legacy ports, slower buses, etc...

By going to non-modular designs, you can make better use of the space. The tradeoffs are big, especially on laptops (increased battery life), and more so in mobile.