julia You are right, but I feel safer using something made by GOS.
I think this would have multiple ways to work out poorly for the developer team. Whichever calendar app were chosen, some people would prefer if it had been a different one. There would be lots of feature requests, including people wondering why some "obviously-necessary" feature had been left out. Any time an upstream calendar app had a new feature, people would complain about any delay in the GrapheneOS team releasing it (and there would need to be delays if there were any security review). Some people would complain about the (small amount of) storage space consumed by whichever calendar app the Graph team had chosen, because they would be installing their own preferred calendar.
Overall, it would be more work, that would plausibly lead to more complaints. Right now each GrapheneOS user can choose a calendar app to suit personal taste, and there is no need for users to battle over which app should be the one that is shipped. I will not be surprised if the GrapheneOS project declines to ship a calendar app for years or forever.
Please note that I do not speak for the GrapheneOS project.