Why do some devices have external power supplies and some do not?

The power supply adds bulk to the device. If you are trying to make the thing small and pretty you don’t want to add any extra bulk if you can get away with it.

Besides what @Lightlyseared said^ Cost.

Actually, the power supply for just about all of those electronics is actually a component inside the casing. That big heavy brick thing you lug around for juice from the grid is an adapter, or otherwise a power cord.

While some devices can run on battery power with enough efficiency and longevity, most devices nowadays have some sort of external method of recharging or staying powered on

@gambitking err…. the power supply unit (PSU) is the big heavy brick thing. A PSU is a device that transforms (literally) the electricity from the 110v/240v that comes from the wall into the 12v and 5v that the components need to run.

If you use a wall mounted power supply there are a lot of UL regulations you don’t have to meet.

An external power supply removes a major heat source from the box, which should help the electronics last longer.

@Rock2 More specifically, the wall-wart has to pass UL since it’s connected to relatively dangerous wall voltage/current. The rest of it doesn’t have to get UL inspection as you note.
Most vendors will contract out for UL-approved wall-warts.

For Engineering, it is a Heat dissipation issue.
For Marketing it is a “Specmanship” issue “My device is lighter and thinner than yours.”

@dabbler
If the input voltage to the laptop is a low voltage , like 12Volts, you can avoid the costly high voltage requirement testing for UL and CSA. That will also help time to market.

I don’t think heat dissipation is that big of a problem anymore.