Why does out require an additional preposition in many cases that in does not?
I am in town this weekend. Next weekend I will be out of town.
When the lifeguard blows the whistle, everyone must get out of the water. When he blows it again, you may get back in the water.
I am in the office. At noon I will be out to lunch.
One wouldn't say "I will be out town" or "get out the water". There are also some cases where you would say "I will be in to town" or "get back in to the water", but omitting the preposition is also perfectly acceptable.
Other positional prepositions tend to require an additional preposition as well, but not "in". You go down to the river, over to the next county, up to the 15th floor and off to the races.
Why is "in" different?