Why ordering systems often fail at scale?

In large-scale retail systems, 𝗼𝗿𝗱𝗲𝗿𝗶𝗻𝗴 is not just about adding an item to a cart. It’s the 𝗵𝗲𝗮𝗿𝘁𝗯𝗲𝗮𝘁 𝗼𝗳 𝘁𝗵𝗲…