Viruses evolve, some quite quickly. The flu isn’t the fastest (looking at you, HIV), but it’s up there. Over time, existing vaccines train your body to fight something that doesn’t quite match what’s in the wild (i.e. efficacy goes down with time). That’s why there’s a different seasonal flu vaccine every year.
They create flu vaccines on a yearly cycle, and a pandemic can kick off in a matter of weeks and months, so if it doesn’t match the preplanned cycle, they’ll have to invest more resources to creating the most up to date vaccine off-cycle.
There’s also the question of disposable income. Top X% have more wealth and income than us, and that excess is basically all disposable. Most of the income and “wealth” of the bottom of the economic ladder is tied up in survival needs, so it’s even less of a possibility than at first glance.