It's the default setting for round robin tournaments that all games start at once.
You can create a schedule, there's a sub-page for that when you create the tournament. But then, you can't distribute the games in batches, like 5 games in Week 1, next 5 in Week 2, and so on. Scheduled games must not overlap in the timeline, the tournament system does not allow for that. Which is plain nonsense, because you can easily manage 5 games in parallel, but 14, yes, that's quite a job and it creates stress for some people. I lost some tournament players because of that.
I discussed that with Sourisdudesert (if I remember correctly...) and he told me that there would be no need for that. Just choose a longer time frame for the games. The stress of having lots of open games wasn't a thing for him. And I think that he had the Arena on priority at that time.
That's why I create my river picnic series in Carcassonne with the maximum time of 30 days. Most games are over in one week, some are not, some still time out (because there are players who try to do 300 time based games in parallel and drown in the flood). For us normal people, it can still get stressful when you have several of games open from the last week and the next Tournament Week swamps you with 14 new games
Robo