Clearence for a trailer storage box is the usual reason .... since the "dolphin striker" sticks down.

Instead of a single carpeted 2"x8" (laying horizontal) for a bunk ... I stacked two ... through bolted w/ galvinized carriage bolts and then carpeted ... (use pressure treated wood and they will not rot, lasting many years ....don't ask how I know to use pressure treated wood ...)

2x10's or 2x12's work well also ... as they provide more surface area ==> more support.

Harry