First off, I don't know why there's any surprise. He already left the Cs once before for contract and also team fit/timeline reasons. This time is a very similar situation, with the team being in a transitional year and also only likely offering the minimum vs. 6m. Sure, he loves the franchise but it's not like he was going to live in Newton after retirement. He put his house up for sale ahead of time, for ****. Not that that means everything, but you have to assume he (and Holiday and Porzingis) knew this version of the team was going to be broken up.
Even though Brad made a token offer, it's clear that the team needs to develop guys like Queta or even Garza and Williams for the next generation of this team. Yes, Horford could have helped mentor them, but he also wants to play. Those minutes would have been at the expense of someone else who might be better for our future.
Secondly, I think fans sometimes underrate other motivations such as, maybe he wants to have the experience of playing with some of the iconic players and teams of the 2000s. Maybe he wants the experience of playing for Steve Kerr. We can argue about how realistic it is for the Warriors to actually be a contender, but you can't discount the fact that when his career is over, he'll be able to say he was teammates with Steph Curry or Draymond.
Let's speculate further and say maybe he wants to go into media after retirement. He'll bring more value to the table if he can speak directly about the Warriors franchise and stars like Curry, Butler, and Green. He'll have a connection to the Boston market and also the SF market, which are two big ones.
But really, I think it's 99% the opportunity to play with Curry and have the experience of being in another iconic franchise. Whether or not they really have any better shot than the Celtics isn't as relevant as the simple fact that one team is going for it, the other is still trying to shed salary for tax reasons.