In my case: I don’t have that many TODOs. For instance, I clean home twice per week, on Mondays and Thursdays. Don’t need to write it down anywhere. I have perhaps 2 or 3 doctor appointments per year (and they are usually within a few weeks in advance, so easy to remember). I don’t have kids (this one could be the game changer, I admit).

Work-wise: sure, I have everything in the work calendar. But i have separate laptops for work and personal life. I never mix them.

If we want to go out for dinner, we just go. Cinema? We just go as well. No appointments. I do exercise at home. We usually travel around 2 times per year (again, not easy to miss)

My wife is 35 weeks pregnant and I go with her to all her OB appts. If we didn't put it on a shared calendar, someone is gonna miss it (though to be honest it'd probably be me, which isn't the end of the world).

We almost never schedule dinners, but we buy cinema tickets ahead of time (or concert tickets, or whatever event). Those ones are hard to remember because the show may be at 6 but doors open at 5 so lots of stuff needs to be coordinated.