why are there so few gas stations in the middle of a large city? (think economics)?

Answer 1

Because the land in the middle of a large city is likely to be very expensive.

Answer 2

Most large cities were begun in the early to mid-1900s when motorized vehichles were relatively few. Buildings were erected to house offices & stores w/apartments above them.
Intracity transportation was mainly bicycles and horse-drawn public vehicles. By the time automobiles became in wide spread use, there was no available land "downtown" on which to build gas stations. Even in those days, to raze a building to construct a gas station was very expensive.

Answer 3

All the above, add:
Downtown buildings tend to be multistory in order to take make room for as much revenue generatin' lease payin' business as possible. Have you ever seen a 10 story gas station?

Also, they're not needed downtown, it serves the purpose to put them on the fringes of downtown where land is cheaper.

Answer 4

Since they are traffic and noise inducers, there are also likely zoning laws to keep them away from downtown or residential areas.