One word: liability
If someone somehow drowns or gets hurt outside the designated area, it creates a whole mess of problems. Lakes, pools, etc are licensed, when required, to have a certain number of lifeguards calculated by the amount of swim space and/or the amount of swimmers. Of course it varies from state to state. The lifeguards at our park district are under regulations so tight it makes the Gestapo look like the Girl Scouts. If you screw up even once, you're gone. If they catch you not checking the perimeter of the pool, even when totally empty, you're gone. I've never seen ten lifeguards in one place ever unless it was a staff meeting.
Lifeguards: An easy job until some blue kid gets you fired.
(and, no, I've never been one but do know the rules they have to live by)