Basically because we don't feel like it.

If you look at the unmanned side of NASA, that's going great. NASA can get amazing stuff done.

The manned side gets political attention, and the nature of current politics makes it a bad kind of attention. Results are essentially irrelevant. Jobs and cronyism are the point.

The overall design of the Space Launch System makes very little sense. We know all too well that solid rockets are a bad idea for crewed spaceflight. Hydrogen is a bad fuel for a first stage. It's horrendously wasteful to use expensive, complicated engines designed to be reused, and then throw them away on every launch. Early estimates were over $2 billion per launch, which in the current age is total clownshoes. The actual costs will be much higher still.

So why are they doing it? Because using all this old, rather inappropriate tech allows them to keep paying the contractors for it. If you gave NASA a pile of money and told them to build a moon program, they wouldn't build this. But it's not their choice.