Aren't "allowed" by whom? If it was economical to replace these reactors they would have a plan for it.

By the US Government, nuclear regulatory agencies, the state governments, and above all ignorant people who are afraid of the word "nuclear" and anything associated with it, because they've never understood the idea that nuclear energy can mean something other than "bombs".

For a new plant type to be built, it first has to be approved by the US government. Then funding has to be acquired, insurance bought, permitting done. Unless the US government makes an effort to clear the way for new construction, all that overhead and cost makes new reactor types a non starter for any for profit business.

They could build more of the older types, but with the unreasoning fear associated with nuclear anything in the US population going on, the cost of doing so is ridiculous... and most of these companies don't much care about climate change anyway. They're about profit.