In the 80s / 90s, computers were very expensive and it was common to only buy exactly what you needed. PCs excelled at this. Everything was an option: Video card (MDA, CGA, EGA, VGA), ram, hard drive, hard drive controller, serial port controller, sound card. If you later needed more capabilities you upgraded whatever was necessary.
As computers became cheaper, upgrading began to make less financial sense. Upgrading a component and paying for it to be installed became almost equal to the cost of a new computer. There also became less to upgrade, onboard sound, network, etc was good enough. The only thing left to upgrade was CPU (but usually required a new motherboard) and video card. Upgradability which was originally a huge selling point became a niche feature.
As integration continued, size, weight, power consumption all became more important than upgradability / customization. Hardware wise, all Android phones and the iPhone are pretty much the same.