This is like asking an open source maintainer what their motive was to contribute to an important piece of software, since it’s going to be hosted on a for-profit entity like GitHub. The motive is that reviewing is necessary to make science work (and secondarily it’s required for promotions etc.) The problem in this setting is that Elsevier is part of the equation and demands copyright ownership, which makes it much worse than hosting on something GitHub. Unfortunately “just stop contributing” isn’t a good answer, because that would throw out the baby with the bath water, and scientists care very deeply about the baby.