It's a dumb rule/law. Congress passes the laws and budget ... then they also have to raise the debt ceiling? Why the two-step process?

Because Congress has different ways to fund the spending they authorize in the budget. They can raise taxes, of course - and have a host of alternative taxes they can turn to. Personal income taxes, excise taxes, corporate taxes, tariffs and duties...they can even impose a capitation if they wanted to. They can also raise money by selling assets - the U.S. government owns tons of valuable property. Or they can borrow the money. All of those different ways of bringing in enough money to fund outlays require Congressional authorization (for the most part), so whatever method is chosen, Congress will have to have a second step to get the money.

So if Congress authorizes a given level of spending that exceeds current estimated revenues, there's no presumption that the difference has to be - or is intended to be - met by issuing debt. Congress has to choose how the shortfall will be funded. They have typically done so by authorizing the Executive to issue more debt - but the Executive can't do that on its own.