SSD’s are built using multiple NAND flash channels attached to a controller. With higher queue depths, the controller can optimize the order of the operations by executing all the operations for a particular channel all at once (even though the operations may not be in the order received by the controller).
Reads are more highly optimizable than writes so the performance numbers for reads go up faster than writes at higher queue depths.
It’s become a de facto industry standard to present performance numbers using a queue depth of 32. Since all manufacturers follow that standard it allows customers to compare ‘apples to apples’.