why do half of plug designs take up multiple spaces on a power strip?

Sometimes it's intentional. To prevent you from using too many high power devices on a single outlet.

It's a conspiracy to bolster the 6" extension cord industry.

So AC devices typically are just a low profile plug and wire. Since they natively run on your houses power source there’s not much going on other than internal fusing which doesn’t really take up that much space.

Most electronics, however, are DC, a lot of electronics are being made as dual voltage. Regardless, this means that the device needs some kind of voltage converter. Manufacturers can either build the converters into the unit itself, or they can build the converter into the plug circuitry and save space on the item the consumer is handling. Consumers have vastly chosen to deal with bulky wires and slim devices.

Because they are designed as a standalone device connected to a single socket.