Walmart and Target already had warehouses, but the end-to-end system you need for 2-day, 1-day, same day shipping, which includes warehouses/trucks/drivers, is not something they had

They were built for people to come to their store, and the website was a second class citizen for a long time.

But that was Amazon's bread and butter. They built that fast-shipping moat as a pretty established company, and the big retailers were caught off guard