1. Costco stores, like many stores, are designed to trap you inside longer and to walk further to be exposed to more buying temptations.
2. Food and toilet paper are placed in the back so you will have to walk past more tempting items to buy.
3. Costco tends to move products around inside the store so that you will walk more to find them on your next visit.

Costco is not alone. Grocery stores put milk and eggs in the back of the store, often diagonally opposite (the longest path) to force you to walk through more of the store.
From your perspective, you would like to get in and out as quickly as possible, but your efficiency is not their goal.
This is what I call “architectural propaganda” – where the building and layout are designed to influence you.
Source: Costco infuriates customers by constantly moving things around the store — and employees say it’s a brilliant strategy – SFGate