And yet my point stands: if you need to do a lot of calculations at your job, you’d be stupid not to have a calculator in your pocket. And if you don’t, then the time it takes to find a calculator will be negligible.
Security and convenience are opposites. You have to decide if you want a local-only manager that is more secure, a sync service like syncthing that you can set up yourself, or a third-party cloud app like LastPass (which has been compromised at least once that I know of).
Personally I just do all my email and banking on my desktop at home, and it’s actually only inconvenienced me a few times over the years.