Can someone explain to me why the advice is to max out 401K before mega backdoor Roth?

Here’s my thought process: If maxing out Roth IRA comes before maxing 401K ( after the match) and Mega backdoor is effectively the same thing as Roth IRA once you leave your company ( which I intend to do in a few years. ). Then why is the common wisdom advice to max out 401K accounts before touching the mega door path? What am I missing here? Thanks in advance