Part of the reason is that the only way to get the acquisition of a major new weapons system approved by Congress is to drastically underestimate the cost. This gives Congresspeople political cover to vote aye. Everyone involved knows that costs will explode later, and by that time the program has become too big to fail. Everyone knows we need a new frigate and it's too late to start over from scratch without endangering national security.

The other reason is that this program is constantly chasing a moving target. The national security establishment is getting increasingly freaked out about the risk of a naval war with China. The original inexpensive European frigate design was largely designed for lower threat environments, mainly European coastal defense against Russia plus gunboat diplomacy in Africa and the Middle East. Now the US Navy needs something that can fight and survive against a superior force in the Western Pacific, so many of the combat systems are getting upgraded. And even with those upgrades it's still dangerously short of VLS cells. If Navy leadership knew in 2018 what they know now then they might have chosen a different approach.