Thanks. One obvious other motivation occurred to me that I didn't mention earlier...
Defensive engineering. This is not trying to set up so that you stomp enemies, but trying to set up your clan so that you don't get stomped by enemies. And this can take 2 forms... a bit of mild engineering yourself to try to mitigate the effect of enemy engineering. Not enough to give stupidly easy wars, but maybe give you a chance against some engineered enemies. (eg going a bit lopsided with x.5s during the March-October period) Or alternatively engineering your roster so you get good matchups - either dodging engineers as in the fairly drastic roster engineering suggested in
this thread. And/or simply organising your clan so there's a higher probability of a good match (if your clan is similar to thousands of other clans you have a good chance of balanced wars; if your clan is weird as hell you're going to get flaky matches)
And I'm also a believer in the "everyone is an engineer" philosophy. If a TH10 with max-ish defences and 10/10 heroes applies to your clan, do you let them in to war? I doubt it, and we certainly don't. Why not? Because you think doing so would hurt you; and rejecting them improves your chances in war. The reality is that (almost) every clan makes some decisions about their roster and what people can build to help their chances in war. The clan doing the defenceless TH11 isn't travelling a fundamentally different road - they've just gone a lot, lot, lot further down that road. Certainly those defenceless are irritating to most people, but it's a pretty blurry line as to what's OK (I've asked a few of the foaming-at-the-mouth-anti-engineers on here what is "acceptable", and got quite variable answers).
So when you ask why do people engineer, often the answer isn't fundamentally different to why you reject that TH10 with garbage heroes. When faced with a crazy engineered clan it might all look pointless and boring - and to you or me it probably would be - but the underlying motivation is similar, and either they don't get bored so easily, or didn't foresee how tedious it would become.