What is the motivation to start a new journal in a crowded field? I've recently noticed a trend of big names in my sub-field starting new journals whose scopes substantially or fully overlap with pre-existing journals, and the motivation is not very obvious to me. To be clear, these are intended to be reputable journals with rigorous peer review, hosted by major publishing houses such as Wiley or Springer-Nature, so I guess it's not a money-grab or anything similar.

To give some more background, my sub-field already has 4 well-established journals devoted to it, as well as >10 journals whose scope intersects with my field (think bioanalytical chemistry vs. analytical chemistry), not to mention the Science and Nature-level journals and the catch-alls such as Scientific Reports. Many of these are highly-regarded society-led journals, with 20+ year histories and the journals all have stable impact factors that range from incremental (1-2), standard (3-5) to relatively high impact (6+), accommodating a wide variety of work.

Thus, it seems a bit mysterious to me why someone would go to the substantial effort and expenditure of personal political capital to create a new journal, harangue your colleagues to submit, convince people to review, etc., when there isn't a clamoring need in the community for yet another journal?
Being able to set editorial policies can have a significant impact on how science is done in the field. Some examples of such policies include: encouraging the use of preprint services, mandatory data/code sharing, transparent peer review process, encouraging replication studies, etc. For the big names who are not satisfied with simply setting the direction of their own labs, setting up a new journal can be another ideal career ambition.