There's a Doom playthrough with John Romero where he talks about waiting until the very end to design the first level; that is, it was the last one he made.
The idea is that by the time you've finished the rest of the game, you're really good at making levels. You have a solid understanding of what makes a passage fair, balanced, engaging, and so on.
The first Doom level is wild. It's really just two rooms and a hallway, unless you go into the third room... Or find the secret way outside, or the rocket launcher, or...
Watching pros play deathmatch in the first level is a trip.
That being true, the point of creating the first level last was also showmanship-- once you've honed your skills on all the forgettable middle levels, you're able to create a top-tier intro (and finale) that everyone remembers decades later.
The idea is that by the time you've finished the rest of the game, you're really good at making levels. You have a solid understanding of what makes a passage fair, balanced, engaging, and so on.