My experience was that both my major employers were dreadful at "succession planning" and the second one had such a bad recruitment process that most people put their head in the sand and didn't think about it.
I resigned from both of them and served my notice, in the second case I gave more notice than my contract required and said something like "I'll leave on 5 April next year" and that was accepted. In my first job I was in an industry with "proprietary secrets" so could have been asked to leave immediately, but sadly I wasn't, but I had lined up my next job taking this probability into account.
I once had someone for whom I was line manager and he resigned, but then came to me to say something like "I'd like to start my new job on 6 February, but I didn't give you my notice in time". My response was along the lines of "I distinctly remember your saying you were leaving a week before that date, and that's what I'll put on the forms I have to complete. If anyone questions this to you, send them to me". He left and started his new job as he wanted to, there were no issues.
I was able to plan on serving my notice both times. I know companies and junior management panic when someone gives their notice because they haven't previously properly considered the possibility. Employees are pawns to be moved around at the will of the employer, and the idea that they have independent thought, plans and options doesn't occur to the employer.