The dealer is giving you an average good pressure - meaning it will provide a decent ride, not to hard and also provide decent load handling - he's giving you an "average" point at which to set the tire pressure.
If you run the tires at a lower pressure you will get a softer ride and less mpg. If you inflate the tires to the max printed on the side wall then you will get a harder ride and better mileage.
The dealer gives you a "middle of the road" pressure that should suffice under most conditions.
In fact, old school shops would deflate auto tires so that when you left their shop you would feel a really comfy ride.
So, "What's the best tire pressure?"
It depends on what you want. I want good mileage and road "feel" so I've set all six tires to the maximum pressure for camping. The first tire set lasted 80,000 miles. The second set were still on the truck at 118,000 miles when I sold it.
The truck felt/road like $hit when it was unloaded but with a couple of tons in the bed it road great. Don't argue with the dealer. The truck post sticker is a rating to give you a good ride and make you like the truck.
The dealer is giving you a "middle" ground.
I'm liking the best mileage and road feel - so I inflate to the maximum. It has worked for me for 100's of thousands of miles.
"Your mileage may vary" as they say.