This always seemed backwards to me. I always though that if you took a 100 watt load (light bulb) and put it in series with say another 100 watt load electronic device that you would pull 200 watts out of your power supply. But in reality the most it pulls is 100 then when the load is added the current stays at 100 but dims the bulb? To figure out which device; bulb or load gets what % of the hundred watts you would have to measure the ohms of the bulb while its hot and your electronic device? Then E=IR it? Do bulb behave as temp controlled variable resistors? Or is there something special about the bulb where its resistance changes in proportion to filament temp? I always thought 100 watts + 100 watts = 200 watts draw. With no real light bulbs to test I can't do this.
Also instead of a 100W light bulb that's illegal can I substitute it with a CFL with a really high CRI that says it's as bright as a 100 watt on the box? Will I get arrested if I plug in a 100 watt filament bulb by the signals it sends to the power company?
I hate to say this to someone with over 800 posts in an Electrical Engineering forum ... but are you trolling? Really?
Because your post, if taken seriously, reveals some really fundamental misunderstandings about the relationship between current, voltage, resistance, and power, and also between such basic circuit concepts as series and parallel connections.
While others have responded in some detail above, all those answers are "assuming" that you have the necessary fundamental understanding of the concepts... but your post, if taken seriously, reveals that you may not.
The short answer to the question in the Topic Subject is: Because the "dim bulb" is connected in series with the DUT, not in parallel with it.
May I respectfully suggest that you review the basics of Ohm's Law? And work out some parallel and series resistor networks, voltage dividers, power dissipation in resistors, and such like problems, just for practice. Hopefully some "lights" will brighten up as a result.
(By the way the "Dollar Stores" around here have plenty of incandescent filament bulbs in stock... you might try checking your local...