Why can't political redistricting be done simply by adding or subtracting adjacent zip codes?

Surely by using census and post office data, not to mention private industry's marketing research, we pretty much know the precise population of every zip code in the nation. Why can't political redistricting be simply done by adding or subtracting adjacent zip codes?

Start from one corner of the state and go north, east, south, or west (depending on what shape the state best lends itself to) one zip code at a time until the needed population for the particular district is met. Then go on to the next set of adjacent zip codes until the whole state is covered.

Dave Reich, Perry Hall