"a) They don't know any better"
Similar games such as tower defence games don't require you to be aggressive.
"b) They know they can win by attacking, but it's more fun to play tower defense"
It's also harder to micro a moving force. Defence is placed at 1 spot and then you kill each wave as they come to your defence, you only need to consider placement of groundturrets, shields and ground AA once before you place it. If you want to kite the chickens you also need to consider how much space you need before the next wave hits you and move out to get enough room.
c) It's I who doesn't know better, and D-FENZ is actually the right way to beat the chickens
I mostly played the harder chickens but the chickens will tech much faster if you sit in a corner and defend and I find it really difficult to fend off the higher level chickens before i have 10-20 Annihilators.
"Godde mentioned somewhere that keeping PAR between 1 and 3 is the easiest way to win. With that in mind, I think it would be interesting if, simultaneously:"
a) It was difficult but possible to win with a very low PAR,
You get surprise attacks in your base, long range chicken artillery, hightech chickens earlier. It sure is dificult. Placing defence is relatively easy comparing it to having to move out, kill burrows and reclaim. It would be hard to say what would be a good level of difficulty for porcing in a corner.
b) It was difficult but possible to win with a very high PAR
Just kill burrows as fast as you can and kill the queen when she comes. If the player aggression rating is high the Queen anger rises alot when you kill burrows. It's difficult to get enough metal if the queen comes at 10 minutes :D.
c) It was easy(er) to win with a neutral(ish) PAR, but difficult to keep your PAR within that neutral range.
Given that killing alot of burrows makes the queen come come early and that chickens tech fast if you play defensively it is generally "easiest" to play aggressive and kill some burrows but not too many.
The problem when you are being aggressive is to keep the chickens from running through your lines or empty areas behind your lines while also keeping your builders occypied with reclaimig eggs(i mostly play chiken eggs).
You might end up with the chickens pushed all in 1 corner of the map and then it gets hard to reclaim the eggs that you kill just outside the burrows without actually killing the burrows and if you are too aggressive you get White dragons that might spawn before you even have a sumo.(Bandit, Outlaw, (jacks morph inbetween), Sumo and shields plus vandals and flak for AA is typically what i use against chickens. Then i have a defencive line of annihilators as a retreat zone against heavy chickens.