my ADHD is acting up to much to read all the text above, so in case it wasn't included: the is a physical limit to frequency in conventional conductors. normally metals have only ohm resistance, which does not depend on frequency and is generally rather low, but for instance in the case of copper a capacitative resistance significantly rises at around 1ghz, which gets only worse. this comes from structural effects in the lattice which form like small capacitors. i don't know the exact details from a solid state point of view, just remember the fact itself from a fiber optics lecture.

long story short: for high frequency (ergo, high data throughout) signals the resistance becomes insane, so you want to have your conductors as short as possible.

another thing that just pops into mind: i remember that for pci-5 the motherboard length is also more limited, due to the same reason.