Maybe they're smarter than you think:
~2 spear fighters denies you more battle points
~non-boosted goods buildings denies you the full plunder value
~TF's medal yield is paltry - again, a good choice if you want the plunderers to go find a 'better' victim (although my favorite is to do crowns from the sleigh builders as that means the plunder gets Zip and their guild gets the crowns)
and last, but not least, why waste hard to acquire 1-one up and Reno kits to age up buildings if they're planning on moving through the eras quickly? Or, if they are the ones that are built from tech achievements, they've just saved a lot of coins/supplies/goods in not necessarily acquiring the more expensive higher era buildings.
It's all about perspective! Sometimes defeating the opponent by making oneself less 'valuable' is far more satisfying than the instant gratification of earning more from your city "now."
Of course it could just be that no one has ever taken the time to explain such things to them as would happen from either a friendly neighbor or a good teaching guild. Pity.