I don't think it's accurate to say that Rupp dodges competition in the marathon. The bronze in the last Olympics is proof enough of that. He went toe to toe with many of the best in the world and came up in 3rd. Yes, the race limits it to 3 Kenyan and Ethiopian runners - but he still took some big scalps that day.
The choice of marathons may be more in line with appearance fees. We don't know how much he was offered by Chicago compared to NY or Berlin. I assume that Chicago offered to pay the most because there really weren't many other big names in the race that year. Based on race choices, I am under the impression that Galen would prefer to run a marathon in the US, which is consistent with many of the other top American runners - and who can blame them. They will get better media exposure winning top US races than if they went to London or Berlin. Let's face it - only the serious track fans are going to wake up to watch the broadcast of a race that's run in Europe 6 hours ahead of EST.
Rupp also "went for it" in Chicago and crushed the field. It was his first marathon with decent weather and he dropped everyone around what - 30K? I don't think that he is afraid of going for it, but trying to drop everyone at the pace they are running isn't as simple as suddenly running 10-20 seconds faster per mile. That kind of gear shift is difficult for the speed they are already running at, made even harder if the temperatures are high/conditions are bad. You can't just throw it down in 70-80 degree weather and expect to maintain that for very long.
Now critics can point to Boston, 2017 - which was hot and won by Geoffrey Kirui with a 20 second burst from mile 24 to 25.
But, my response to that is, Rupp came into that race unhealthy. And despite not being at 100%, both he and Kirui HAMMERED the last 12K in some pretty terrible weather conditions. Rupp and Kirui were neck and neck the whole way until the 24th mile. Kirui put on a surge that opened up a 15-20 second gap and that's what won him the race. They ran low 5s from 25 to 30K (down the hill into Lower Falls and up the hills to Newton City Hall) ... then they ran sub 5s from there, up the next two hills, including heartbreak, to just before Cleveland Circle at 35K. That's not easy running right there. Kirui then averaged 4:41.9 pace (14:34 5K, aka the exact WR pace) over the next 5K to 40K (just before the 25th mile where he had broken away from Rupp) while Rupp averaged 4:48.7 pace over that same distance. Considering the day and his health, 2017 was an incredible race. If he can stay healthy for 2018, he'll be a contender.
And overall - a win in Boston for any of the Americans will be HUGE. Look what it did for Meb (I get it, it was after the bombing). But that win by Meb, coupled with the bombing itself, has put Boston back on a pedestal of sorts. Everyone who runs knows about Boston. Those who REALLY follow the sport may know that London attracts some of the faster runners, but your average everyday person probably doesn't know that, and more importantly, won't care. If Rupp were able to win in Boston, then this would be something both he and Nike can broadcast for the rest of his life and it will have 10x the value of a win in London.