Hey! For convenience, mostly. We used to depend on Mixapanel for analytics and MongoDB was a good drop-in replacement with only 20-30 extra lines of code. I agree that Redshift would be a better choice for complicated queries afterwards but the only queries we run on the event data is looking up 2 events with the same "training_id" and calculate the time difference between their timestamps for billing purposes.
The "important" events for data science are still stored on postgres (i.e. logins for checking if a login was malicious)
The "important" events for data science are still stored on postgres (i.e. logins for checking if a login was malicious)