Why did Korach make his argument only after the incident with the spies?

Because through that incident, we learned something: That living a spiritual life in the desert is nice, but only when the goal is to bring it down to action on the ground.

If so, Korach said, we’re all the same.

Because in physical activity there is no higher and lower. What makes a tzadik like Moses or Aaron so far beyond everyone else when they do the same actions?

Some people see spirituality as the focus of life. Others say we’re here to get things done.

But there really is no contradiction.

Actions count, but you need them to shine.

And when do they shine?

When you make spiritual matters the focus of your life.

And for that, you need a Moses and an Aaron and all the tzadikim. To lift you to a whole new level where you can shine.

Likutei Sichot, volume 4, Korach.