Local encryption came up countless times already. These days most operating systems already provide Full Disk Encryption. If not, there are other ways, but not as convenient, since one would have to open a container first. (But even these things can be automated.)
In any case, the premise is that if someone already has access to your machine, you most likely have bigger problems. If you want plausible deniability, local encryption won’t help either. If you want to share your computer, use different accounts. If you want to share a computer and use a shared account, don’t even mention the words security, encryption, and safe to me.