Sure, today it seems pretty odd to carve a face into a pumpkin, of all things. But today any object humanly imaginable -- of any size, shape, color, shininess, sparklitude, and luminosity -- can be manufactured for pennies in China. So you'd have to be pretty crazy to just spontaneously decide to waste your time carving vegetables.
But think back to what it was like for people at the time when the jack-o'-lantern tradition arose. Just because they were peasants with no access to the magic of cheap Chinese manufactured goods didn't mean that they didn't want festive decorations for their holidays.
And think what they had to work with: dirt, vegetables, maybe some rusty tools if they were lucky, and candles. Under the circumstances, carving vegetables into lanterns seems like a perfectly obvious thing to do, hardly requiring any kind of excuse or explanation.
Note that I mentioned carving "vegetables" and not just pumpkins. That's because I've always liked reading about the history/evolution of holiday traditions (see here), and I'd read at some point that instead of using pumpkins, a lot of people in centuries past would carve their holiday lanterns from turnips!
Imagine my surprise when I discovered that this tradition still exists! In fact, my family and I were even invited to participate in a "Raebeliechtli-Umzug"!! (Online translation tells that me means "Turnip-moving," but I think would be more accurately rendered as "turnip-lantern parade".) (Also note: my Austrian friends claimed the lanterns were rutabagas, but sadly my ignorance of the subtleties of root vegetable species prevents me from telling a rutabaga apart from a turnip.)
Here's what the Raebeliechtli were supposed to look like (shown on the wagon that led the parade):
And here's what mine looked like:
Now I know what you're thinking: It doesn't take a vegetable expert to tell you that's not a turnip. But I couldn't find any turnips! Note I also carved a face into it, which you're also not really supposed to do, but old habits die hard. Anyway, now that I know what they're supposed to look like, we'll do it better next year.
One of these two Raebeliechtli is not quite right
As for the story of the Turnip-Lantern Parade? Even Wikipedia doesn't seem to know. My Austrian friends said that the tradition exists in Austria as well, and may have something to do with the harvest. (Maybe kids were motivated to come up with creative alternate uses for turnips to avoid having to eat them...?) Who knows?
But now it's a tradition. And as long as I'm living with kids here in Raebeliechtli-land, I'm going to follow it!