The guy the journal editor talks to has sometimes a different agenda. As he does not have interest in reviewing the papers, he isn't interested in the topics. He's often interested in advancing his career and building his network, and a move to do that is "review" papers for journals. Otherwise he would review the papers on his own.
Boycotting would run counter to his own goals (while the guy who got the task of actually reviewing the paper has interest in the topic and would like to boycott journals but is not in the position to do so).
The guy the journal editor talks to still has limited minion resources at his disposal, and he still has the option of caring about the future of academic publishing even if he isn't interested in the topics of the papers.
Boycotting would run counter to his own goals (while the guy who got the task of actually reviewing the paper has interest in the topic and would like to boycott journals but is not in the position to do so).