Because:
*the DSL modems DSL providers send out these days already have a wireless base station built in.
*I have a hand held device with built-in wireless adapter
*guests with laptops appreciate the wireless connection
*this is a huge house and it doesn't make sense at all to string wires all over the freaking place if everything is already in place for a wireless connection
*full 100MB duplex is limited to certain lengths of wire. Signal also degrades where you can't string them properly, i.e. sometimes you're forced to run them parallel to powerlines.
Did I forget anything?
Mice nibble at the wires.