Why are these cost overruns an issue? The solutions our politicians are putting forward now are going to cost taxpayers trillions and just go to some vague idea

Why are these cost overruns an issue? The solutions our politicians are putting forward now are going to cost taxpayers trillions and just go to some vague idea of green energy. I’d much rather throw that at nuclear research and building nuclear plants, as opposed to some slush fund that will only line insiders pockets.