it is surprising (or not?) that there is such a vast gulf in terms of automated tooling between the semiconductor world and pcb routing world.

i guess maybe there are less degrees of freedom and more 'regularity' in the semiconductor space? sort of like a fish swimming in an amorphous ocean vs. having to navigate uneven terrain with legs and feet. the fish in some sense is operating in a much more 'elegant' space, and that is reflected in the (beautiful?) simplicity of fish vs. all the weird 'nonlinear' appendages sticking out of terrestrial animals - the guys who walk are facing a more complicated problem space.

i guess with pcbs you have 'weird' or annoying constraints like package dimensions, via size, hole size, trace thickness, limited layer count, etc.

It's the opposite; semiconductor design is so full of constraints that it's difficult to do manually. Usually individual transistors and gates are laid out manually, then manufactured in a test chip, and empirically analyzed in every way possible, then those measurements are fed into the router, and it creates a layout of those blocks that allows enough signal propagation to work at the high speeds used in modern semiconductors.

Semi by hand got out of hand (HA!) in the nineties. There is simply too much work for humans (millions of transistors) so we swallow performance hit. Synthesis puts stuff together from human optimized basic building blocks. Same reason FPGA tools quickly advanced from schematic input to hardware description languages.

With PCB its all still quite manageable, even something like whole PC motherboard is easily doable by two-three EEs specializing in different niches (power, thermals, high speed digital design).