People are rebuilding working things from scratch with AI just because they can.

AI eroded the greenfield advantage of rewriting a project from scratch because a rewrite generally meant that the code went through the fingers of the new developer. With AI that advantage is gone so you are better off incrementally improving the old code base to minimize overall maintenance costs and as a result reduce the headcount needed.

AI lowered the cost of rewriting which means that more shitty systems ship because the rewriters don't run out of money before the rewrite is canceled. But it doesn't the solve the underlying problem of the people in charge of rewriting not understanding what features in the old system were actually important and which had to go.