Why do so many teenagers in the US pick up several jobs?

When I say pick up several jobs, I don’t mean due to a dire financial situation (although that is also an important factor, I guess). I mean, how come so many teenagers in the US become employed in their mid-teens as waiters, volunteers, shop employees? how is it not concerning or exhausting to work at a young age, and why is this so crucial to university applications and such? for example, in Europe (or at least where I live), it’s quite uncommon and teens tend to become employed in their late teens and get accepted to universities / colleges based on merits .

^ obviously I cannot speak for other countries where this might also be the case, I just hear about it primarily from the US.