When I first started playing, I didn't even consider NPCing whatsoever because I didn't want to miss any of the plot, treasure, or character growth that comes with PCing, so I definitely understand that mindset for a lot of people. Nowadays, though, my characters have all the skills they need, more than enough treasure, and enough plot experiences that I'm not hungry for more. Every event I PC I find myself thinking "Man, I just don't get the same fun out this as I used to."
NPCing now, though? Now that's my jam. No stress over dying? No stress over treasure? Tons of chances for me to wear a variety of costumes? Tons of different roles rather than having to stick with the same thing all weekend? Tons of fancy makeup that I get to play around with? Cheaper/free event fee that appeases my cheapskate nature? Sign. Me. Up.
Here's the thing, though: A huge amount of my love of NPCing is because I have always had a ton of freedom with it in the chapters I play in. I like to be able to think up unique and engaging characters, monsters, encounters, mechanics, props, etc. and make them happen, and even when I wasn't formally on the plot team of a given chapter, the plot staff knew me enough as a player to give me a LOT of freedom when I joined their side for the weekend. Of all the games I have NPC'd while not actually on the plot team, I can remember being given more or less "honorary plot staff" status for the event. I was given big combat cards, I was given important RP roles, etc. If I was going to an event with the knowledge that I'd be just be grinding all weekend as waves of unnamed, unimportant, constantly-respawning lackey roles wearing a colored bedsheet, I wouldn't NPC.
But that's the thing: New player NPCs tend to get relegated to the most menial roles simply because they are still learning all the rules and the plot team can't usually rely on a new player being able to handle anything more complex, and this tends to make new player NPCs have the most boring, uninteresting first impression of NPCing possible. They get to play the smallest, simplest NPC cards on the field while watching unique, interesting PC characters and big plot NPCs from a distance, dropping their desire to keep on NPCing when they could simply PC and escape their life of NPC drudgery. As a plot person, I know I'm guilty of giving newer or less-experienced NPCs these smaller roles, and while a part of me feels really bad about it every time I do, it's often just sort of the nature of the beast when it comes to running an event.
"The plot staff should just give them more meaningful roles, then!" I hear you say, but it's nowhere near that simple. For an event to go as planned, the important roles usually need to be played by the people that are able to achieve the desired result. Being a good NPC takes a lot of knowledge of not only the game rules, but also physical combat prowess, game design, social skills, campaign setting information, and so much more. In an ideal world, NPC crews would probably be comprised entirely from veteran players, but for most games, that's probably never going to fully happen because there will always be veteran players that would rather keep PCing (and there's nothing inherently wrong with that).
Let me tell ya, though: there's nothing I love more than when up-and-coming plotter players come to me to teach them my "NPCing secrets" so that they can become better plotters themselves. I love to explain my thought processes about how and why I do the things I do when I run plot, and I love to see other players honing their own style as NPCs. Learning and growing as a PC is a way easier route than learning and growing as an NPC, but man, the latter is so rewarding in a way people probably don't even realize.