Because it is quiet. I don't have to talk to anyone.
I spend most of my days being mentally and emotionally available to almost 100 people- most of them under 13 years of age. Don't get me wrong, I love my job, but at the end of the day it is pure joy to be able to just not talk (or even listen, for that matter).
Walking is good exercise, but it is often social. Cycling is less social, but on the tandem I still have to communicate to kiddo constantly. In the pool, my ears are full of water and my glasses are in the locker room. The world is effectively shut out, and I can slide through the water with nary a care.
Swimming is easy on the joints. I can swim for half a mile (22 laps) and my knees don't complain the next day like they would if I had jogged or played tennis.
When I swim, I don't have to concentrate. I can establish a rhythm and just keep going. I can think of songs, psalms, prayers, poems, etc. and nothing interrupts to get those things out of my head. I don't even have to count laps because my watch does that for me. (The only thing I have to remember is to breathe out through my nose and in through my mouth!)
The pool is cool. I can work out, sweat, and come out smelling clean like faintly bleached laundry. And, I can shower at the club and not have to shower at home- saves money!
Kiddo loves swimming. She has been fascinated by water since she was born. She has had years of swimming lessons, and is like a little mermaid in the pool. She sometimes swims laps with me, but more often finds a friend and does her own thing.
I think this Summer of Very Little Television is going to work out quite well...