It is due to the saturation voltage of Darlington pairs, which is normally 1 V.
With a load of 1A, the device consumes 1 W. Quite a challenge for cooling with higher currents and multiple channels and wastes a lot of energy. Darlington topology is simply not a good solution for switching applications where low saturation voltage is critical. The problem is exaggerated at low voltages when the supply voltage is of the same order of magnitude as the saturation voltage.
MOSFETs have no saturation voltage. They behave like a resistor and their channel resistance can be extremely low. Less than 0.001 ohm is possible. With a load of 1 A, the device would dissipate 1 mW, which is negligible. In fact, daringtons require more input power to be fully opened.
MOSFETs would work for you, but the circuit needs to be redesigned. Any pin-compatible drop-in replacement for ULN2003 with MOSFET switches. There are many integrated switches with similar parameters but different pin assignments.