Because the interiors were fun to have shootouts in, and interiors also meant Burger Shot and Cluckin Bells which means full health for 1 dollar only.
I don't get people's fascination with interiors; a few (working) food stands should have been in 5 but what could you do in past GTA interiors that was so fun that people miss them so? I think Rockstar need to add at least a couple of interiors that have naked pics of Niko and CJ for people to jack off to lmao. Just kidding (f*ck you).
Also, I really miss food stands, I laughed so hard the first time I heard "wanna try my wiener?" in IV.