Because logging is persistent. Once I have good logging implemented for a section of code I can always enable it and see what's happening every time the application runs.

Debuggers are one-offs. Running a debugger (and rerunning the application) every time I'm suspicious about something, or need confirmation, is more effort.

Good logging is having an always-on debugger that's enabled for everybody at once. It's an investment that pays off.

Yes, I know how to use a debugger, but rarely has it been more valuable than good logging.