Why don't hall effect current sensors exist for low currents?
Hall effect current sensors measure the magnetic flux generated around a conductor carrying current. As such, the sensitivity is limited by the noise floor due to extraneous magnetic "noise" in the vicinity of the conductor.

This can be overcome to varying degrees by concentrating the magnetic flux due to the current carrying conductor, by a fairly simple means: Pass the current to be measured through a coil surrounding the hall effect sensor. 

For example, section 12.1 of the Melexis MLX91206 linear hall effect current sensor datasheet illustrates the use of a coil for measuring small currents:




  Low currents can be measured with the MLX91206 by increasing the magnetic field via a coil around the sensor. The sensitivity (output voltage vs. current in coil) of the measurement will depend on the size of coil and number of turns. Additional sensitivity and increased immunity to external fields can be gained by adding a shield around the coil. The bobbin provides very high dielectric isolation making this a suitable solution for high voltage power supplies with relative low currents. The output should be scaled to obtain the maximum voltage for the highest current to be measured in order to obtain the best accuracy and resolution.


In practice, so long as the design can tolerate an inductance in the current path, the MLX91206 works well enough down to 100 mA current for full-scale output. When measuring supply rail current, this can actually be leveraged to added advantage by using the inductance for ripple suppression, "for free".



Conjecture: It might be worth exploring whether a non-rectangular (toroidal) coil provides better extraneous magnetic noise attenuation than the rectangular form - perhaps even lower currents can then be measured.