If the framers wanted to require that Congress do this stuff, why insert "have the power to" unnecessarily? There's no need for that phrase.
As another point against this being an obligation, if you look at the list of enumerated powers[0], you'll see some that are clearly not obligations. Congress doesn't have to borrow money. They don't have to maintain a navy. They didn't have to establish post offices. They didn't have to create a federal court system. (Granted, it makes a lot of sense for them to do many of these things, but it feels wrong to think that they're required to.)
[0] https://en.wikipedia.org/wiki/Article_One_of_the_United_Stat...