You are planting your pot too close together. That's why you have spider mites. If you spread the marijuanna out a little bit it gets enough airflow that the spidermites don't actually like living there.

Also why you can't get rid of them. Your shit is too close together.

I live in a spider mite infested area and I've only had them twice and they actually just died off naturally when the summer hit and the humidity went away.

You can honestly remove a lot of your pest issues by understanding what they prefer to live in and making that a non issue.

Now for the commercial grower they want as much money as they can make per square inch, so they pay to remove the issue. (don't worry, I'm also one of those, but I do my dense planting in the winter cause I sell plants to other people). But for the home grower, just opening up the space and some good pruning can give the airflow that makes spidermites go away.