I call discrimination.
Digital transistors seem like they might be useful once in a while, but probably not all that often. There sure seems to be a lot of makes and models of them, though.
Why no pre-biased LEDs for the popular voltages - 5V, 3.3V? You would think it would be useful and easy to manufacture. But nope, no one ever did it.
Why not do you think?
Two problems, come to mind.
One is that it will potentially (ignoring some smart, inbuilt PWM or something), make the LED run hotter inside, for the same current. that would be a bad thing. As they don't seem to be able to (or like) dissipating heat.
The other thing, is that people (in their specific application), want to also choose a brightness (range) for the LED. A pre-biased one (e.g. a built in resistor) would only have one brightness level. Unless PWM brightness modulation or something else was done.
tl;dr
The external resistor, also allows the designer to choose a suitable brightness level and/or have it automatically change with ambient light level via a light sensor.