Why the hell I cannot mark events completed?

I use gcal as a hub to track all my life so it is linked with all the apps I use. So a task ,from let’s say todoist app, appears as an event on gcal that I cannot tick it off. In the end everything gets messy.

I really love the app, but why not crossing the events off? I mean what’s the harm?

Kalantor about Google Calendar: Get Organized, v2.16