What is the motivation to start a new journal in a crowded field?
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.