I could go on and on as to why. All I can say though is that I imagine it was a "simpler" time then.
Also, both of these projects were private sector construction, not the government doing it.
The MGM Grand was built in 22 months, DHS Legoland is still under construction 15 years after they started.
But the government can make things happen if they want to: The replacement for the 35W bridge in Minneapolis was built under a design-build contract in 13 months. That was 'soup to nuts' including debris clearing. It's just that so many people have their hand in the pot, the government has no interest in streamlining the process.