I am confused.

Iran knew USA would come along one day, and they knew the max capability of the bombs they would drop.

So why did they not go a lot deeper/reinforce to a level where the b52 payloads cannot reach.

How do we know they didn’t? I’d be surprised if this is actually one and done.

The GBU-57A started service in 2011. Fordow was completed in 2006, Natanz somewhere before 2002, so there's a good chance they did not know that kind of bomb would be available.

While they could've said "let's just assume there will be something X times stronger than anything known", it would also have increase the price to build these facilities by a factor of X+Y.

I'm no expert, but I imagine once you have your centrifuges up and running, you don't want to continue setting of blasts nearby to add another sub-level to your plant.

pretty basic - they know they have no chance of hiding from the US. so they go plausibly in reach but outside of being casually bombed by missiles from israel. the moment they go deep US will go after them hard.