not at all
in many cases the only replaceable components in a laptop are the ram in the hard drive, both of which are smaller than the "normal" counterparts
Correct; only the RAM, hard drive, and sometimes the wlan card are of a standard form factor. Everything else is usually custom-designed to fit in it. This lack of standardization for everything makes it impossible to really 'build' a laptop. The closest you might get to building one is finding a gutted chaise at a thrift shop or something and just replacing the big components to make it work.