To change speed, freestylers don’t just change their effort, they...

At slower speeds, there are large gaps in propulsion as swimmers are more patient in the front of the stroke.
In contrast, sprinters move the arms in opposition.
This minimizes or eliminates any gap in propulsion, so even if swimmers are creating the same amount of propulsion per stroke, they’re going to go faster because they’re creating more propulsion.
When they can control their timing AND their effort, they’ll be much more in control of their swimming.