The transistors need to "saturate" when used as a switch. Then the base current must be 1/10th the collector current.
The current gain (a fairly high number) of the transistor is called hFE and is when the transistor is a linear amplifier, not a switch, and its collector to emitter voltage (Vce) is at least 5V as listed on its datasheet.
In an H-bridge, two transistors are turned on. Each one has a saturation voltage loss. The max saturation voltage loss is shown in the datasheet and is about 1V at 600mA for each transistor. Then with a 9V supply and a total saturation loss of 2V the motor gets only 7V.