Because there's one in the post office already- a common thing in Japan too. The island has it, so you can deposit any bell bags you get after selling items to that kid, which may push you over 100,000. Thus in Re-Tail, she puts them in bags, so you don't need one, just like in Nook kids shop.
They should have a locker and the ABM at retail. It would be easier for visitors to sell items. No walking back and forth on your pathways. Or taking your items. They go to retail sell everything and than you end the session on turnip or premium fruit days.
I wouldn't mind the lack of an ABM if we could hold more bells. I hate that holding more than 99k takes up inventory space, and that even with inventory space (and using letters) you can only hold 2.5M :/