Why do some PCB manufacturers(pcb manufacturing services) reject gerbers due to missing solder masks

Usually PCB should have 2 sides soder mask if the layers are over 2. But why so some PCB manufacturers reject gerber due to missing solder masks? During you do the cheap pcb manufacturing this small things will often annoying you it often happens in single layer pcb design.

Last time I saw some engineers post  questions about this issue:

"Thanks for your circuit board manufacturing order on our website,but in your file missing the soder mask file   pls help advise this issue before proceeding by tomorrow latest ,tks ."

"Noticed there wasn’t solder mask on bottom copper layer based on your design ,is it correct and can we proceed for production as it ?"

"I changed the board setup to be a 1 sided board (so the back Gerber files are not produced) and sent it again and told them it is 1 sided. PCB manufacturers  rejected it again for the same reason. Per other discussions, I told them to just put mask on the whole bottom side and they said no, to fix it on my side and send it again."

But for those best pcb manufacturers why they didn't understand about this issue ,We would like to suggest like this :

Solder mask is physical substance which is applied to the board. It should not be applied to pads or (larger than very small) holes. The mask layer gerber file looks empty if the mask is applied everywhere. Where there is colored area in the layer the mask is not applied. If your board has no THT components or other through holes or CNC routed holes and has only very small vias the layer can be empty. Each pad and larger hole should be visible in the mask layer gerber.

If you want to leave the board without the mask and send them a gerber file which is completely colored they might think that it was a mistake. If that’s the case then just tell them to not to apply mask to the bottom layer.