But it is.  2 factories with adjacency bonuses provide +60% production (+25% from each one plus +10% from adjacency).
Add a third with adjacency bonuses and it becomes +120% production (+25% from each one for +75%.  3 adjacency bonuses means an extra +15% on each factory.  75+15*3 = +120%).  
Arrange 6 factories in a hexagonal pattern around 1 factory and you get:
7 factories * 25% = 175% (base factory production ignoring adjacency)
Central factory gets 6 adjacency bonuses for an extra +30%.  
6 of the outer ring factories gets 3 adjacency bonuses for +15% each.
You're looking at 175+30+6*15 = 295% production.
 
Now add in raw production boosts from starbases:
Surround your planet with 4 economic starbases, and that's a +40% boost to RAW PRODUCTION.  Add in the interstellar government line for another +60% boost in raw production, and we have a +100% in RAW PRODUCTION.
The factories MULTIPLY the raw production.
So imagine the pop of the planet was 6, for 6 raw production.
(6 *2 (raw production bonuses))* 2.95 = 36 production.
In other words, 7 factories and 4 economic starbases with the interstellar government line will multiply production on a planet by a MULTIPLE OF 6.
Please do the math.