[Suicide Junkie]

If 'labor' were a non-portal resource then you'd be forced to locate your major shipyards at large population centers. Hey, presto, more realism

P&N does this, and I'm sure proportions has something to that effect as well.

In the latest P&Ns (2.5 PBW), spaceyard rate is set to 10% for 1M people, and goes up 1% for every 25M people after that.
Effectively, Homeworlds start with about 175% of normal build rate, but colonies of 100M are stuck with 14%, and struggle to build even the cheapest facilities, normally taking 6 months to a year each.
At the other end of the scale, Sphereworlds get about 25x more build rate than normal
Besides the fact that it would take forever to build the spaceyard on a colony, you'd get little return on your investment.