It's a fair question and I think anyone that has paid close attention to CGC's practices over the past few years doesn't trust them at all but will continue submitting, for various reasons.
The corporate gatekeepers in the mechanical errors thread will defend CGC to the death because they earn a living exploiting CGC, and that's completely fine, though annoying.
Some continue to submit because they just like having their books slabbed and there really is no legitimate competition. Their closest competitor has been caught in their own scandals and incompetence.
Neither of these reasons require trusting CGC. Trusting any corporation to be anything but driven by its bottom line is misplaced idealism. At this point, the market has maintained that CGC is the leader in resale value and collectible desire. That will change at some point, as all things do, but no one knows when so the train rolls on.
I legitimately thought that them selling grades with no doubt that's what they were doing was going to be the beginning of the end but here we are with no one caring.