To create the very first jump gate connection, between Amarr and Hedion, took just over fifty years.
Gates are connected twins, and after one is installed in a system, our ancestors needed to travel by cryoship to the system where they wanted to make the other part of the pair. This costs considerable effort and time, and crews willing to devote their lives and that of their children to the task. Cryoship technology improved, but more than roughly 10-15% of light speed is not achievable. Also the production of gates takes a lot of resources and devotion - even after our forebears arrived in Hedion, it took them two years of dedicated work before they could send a signal back.
Keep the tremendous effort needed in establishing gates in mind, and cast a look at the gate link network. It looks to me tremendously inefficient with many redundant links. The same number of nodes (stars) could be much more efficiently connected - with less links - while still keeping overall travel time small. Mathematicians love this kind of optimization problems.
So, I think that the reason why we have our current set of links (barring of course Pochven) is because there was a mix of links established by the Empires, and links that were still present. Just as we discoved the first (unpaired) gate in our system, I surmise that we must have discovered also paired remnants, maybe leftovers from the Talocan, these master spacetime weavers.
While in the Amarr system, I am also meeting with some mathematician friends of mine, to see what their "network theory" has to say on the matter.
