Because wifi is ubiquitous and everyone has an old cell phone laying around. If you need something the size of a credit card, who the hell are you? Who are you

Because wifi is ubiquitous and everyone has an old cell phone laying around. If you need something the size of a credit card, who the hell are you? Who are you spying on that notices a pocket sized device hidden in a room? You can probably afford to build your own hardware. Almost everyone that needs to bug a room is either 1. Catching their partner in an act of infidelity or 2. A professional with money to spend, and isn't spying on a nation state.