They shouldn't even bother building capital ships, I can think of four solutions that are far more effective.Massive fleets made of nothing but bombers
It's always bothered me how easily a capital ship can be taken down by bombers, but that's the way the ball bounces. Send 100 bombers at a sathanas from behind, 'nough said.Unmanned missile warfare
Take a meson bomb or something stronger, and put two separate engines on it. One engine is used to warp into a system, the other to make an intra-system jump. Send a dozen of them at a capital ship, and you've won the day.Junk/Asteroid warfare
Collect the debris of old capital ships, place a massive engine on the back, and ram away.Dyson Sphere-ish defense
Create a perfect sphere that can be placed over a jump node, anything warping in will be destroyed instantly.
As far as the whole "but the Shivans were doing this
" argument goes.... We have no knowledge of their society. Any attempt to understand such an unusual species is doomed to failure.
Think of it like 9/11. Why haven't the US built another World Trade Center at that spot? It would carry too many bad memories.
Firstly, construction in the United States has gone at a ridiculously slow pace since the '60s or so.