There's less energy in the wind after it leaves the turbine (by necessity, energy is transferred into the turbine to make it turn). If you were to expand the air moving at 15mph to the same size as the inlet its' velocity would decrease in the same way that funneling it down increases that velocity. Plus losses from viscosity and turbulence.
Technically speaking, you could add a second stage turbine to it but you'd be getting a lot less energy out of it and it's probably not worth it. Something similar is done in the natural gas turbine world with combined cycle turbines, where a natural gas turbine is operated at very high temperatures and pressures, producing a lot of energy, and the exhaust (for comparison: the air/gases coming out of it) is still hotter and higher-pressure than atmospheric air. This exhaust is fed to a steam turbine which manages to capture some more energy out of it on its' way back into the atmosphere, where it leaves lower-temperature and lower-pressure than it did from the gas turbine.
Also, for any real-life physical system involving airflow, even if you had perfect turbines that cost zero dollars, everything creates viscous drag with the air that lowers velocity/energy/etc.
Technically speaking, you could add a second stage turbine to it but you'd be getting a lot less energy out of it and it's probably not worth it. Something similar is done in the natural gas turbine world with combined cycle turbines, where a natural gas turbine is operated at very high temperatures and pressures, producing a lot of energy, and the exhaust (for comparison: the air/gases coming out of it) is still hotter and higher-pressure than atmospheric air. This exhaust is fed to a steam turbine which manages to capture some more energy out of it on its' way back into the atmosphere, where it leaves lower-temperature and lower-pressure than it did from the gas turbine.
Also, for any real-life physical system involving airflow, even if you had perfect turbines that cost zero dollars, everything creates viscous drag with the air that lowers velocity/energy/etc.