Why so many Union Stations?

Just curious why there are so many Union Stations...

Cause a lot of stations were shared by more than one railroad.

Not only were they shared they were built and owned by these RR's. LAUS in Los Angeles was built in 1939 with money from Union Pacific, Southern Pacific and Santa Fe.

Often the station was "owned" by a company which was itself owned by several railroads. Washington's Union Station was the property of the Washington Terminal Company which was partially owned by the PRR & B&O Railroads.