Just curious why people buy these seemingly over-priced DC spindle motors like this:

https://www.automationtechnologiesin...or-and-driver/

They seem to be on a large portion of mini / benchtop mills and stores like Little Machine Shop seem to think that they offer "tremendous low end torque". They essentially imply that they offer the same low speed torque as motors with a gear reduction but without the gears.
Are you specifically referring to the brushless DC motors like the one you linked? I mostly see those type brushless motors on machines that either previously came with brushed variable speed DC motors, or similar designs. Mini-mills, mini-lathes, PM-25/G0704 type machines, etc.. I don't generally see them on machines that had induction motors and gearboxes.

Obviously, no gears are better than plastic gears if you get the same torque but is this really the case? LMS offers no info of what sort of torque they deliver which seems odd if it's such a selling point.

The 1.5hp variant sold by Automation technologies only offers 1.7ft lb of torque which suggests they offer no more torque than any other type of motor with equivalent power. So why would anyone buy one when they are at least 2 to 3 times more expensive than your average ebay 4 pole 1725rpm induction motor plus a VFD?
A DC motor, brushless or brushed provides maximum torque at 0 rpm decreasing about linearly as rpm increases. A VFD controlled AC motor offers constant torque below the base speed. So if the brushless and AC motors have the same torque at say 1800 rpm, the AC motor will have less torque at lower rpms. So, for low rpm use you would need a much larger AC motor to have the required torque, or gear it down to preserve rpm and thus torque. So the benefit is the high torque at low rpm and less complexity by not needing a geartrain.