It might be a desire to be alongside the MIT/Harvard laboratories- and to be close to the AI/tech research they're doing.I don't get it. Is Amazon trying to hire co-op students? Why is it a consideration that MIT and Harvard are nearby? MIT and Harvard grads for the most part (i) aren't from Boston area anyway (ii) can move anywhere if the money and the other conditions are good. It's not like their grads have an overwhelming preference towards staying where they are once they finish their degrees. We're talking about super ambitious unmarried 22 year-olds.
I also wonder if another location might be in South Boston near the convention centre. Still a lot of land to develop there as well.