This is why. The platforms are underneath
Penn Station, between 7th and 8th. A Moynihan Station west of 8th would only touch the very end of the platforms (the West End Concourse, part of Moynihan, is in green). This means each platform would only have one point of access to Moynihan, leading to extreme crowding every time a train is departing. It's chaotic enough right now.
For this reason, Moynihan can't be a
replacement for Penn. It can only be an addition that expands capacity.
Ten years seems like a long time, but Penn Station is one of the most congested, crowded sites on earth, rivaling spots in Tokyo. It takes time to come up with a plan that works for everybody, funds to build it, and a long time to build a giant new station when thousands of people must pass through daily. It will take even longer if the 10-year plan passes and the Dolans bring their inevitable lawsuit.
There's also the problem of connections. If Amtrak builds their Gateway tunnel to bring another two tracks beneath the Hudson, where do those trains go? It's been suggested that Penn could handle these trains already, if it had bigger concourses and better management. I'm hoping for a connection between Penn and Grand Central, which would be the key achievement of the entire NY rail system.