Being near an emergency exit improves considerably the survival chances after an accident. Why not put one exit at every 5 rows?
Doors cost (and weight) a lot more than plain aluminum panels. And the exit row on larger planes usually consumes one row of seats. So lets say we are on a 767 and take out every 6th outer row, that's 5 extra exit rows and 20 fewer paying seats, or $6,000 less per flight which quickly adds up to nearly the cost of the aircraft itself over its service life. And that's quite a lot of money for something that will very likely never be used. Remember that exit doors only help if the plane crash-lands in a mostly upright position. Severe crashes often add large exits by splitting the fuselage open, and lawn-dart mode accidents don't leave too many survivors.
The flying public has indicated that they are very interested in ticket price, but somewhat less interested in the safety factor. The current accident rates appear to be acceptable to those paying for the service.