It's because the weight of the vehicle determines the amount of pressure needed in the tire. The tire manufacturer has no idea what vehicle you are putting the tires on.
at work we have special tire pressure calculators that will alter the tire pressure if you go with a tire that's different than the original.
for example, my truck is 35psi with the original 265 70 16's. now i run 305 55 20 trail graps that are 65psi max, but only run them at 40psi. at 45psi they ride like a bag of dicks. if you went 65 like the tire says, have a bumpy ride.
on the other hand, if you're towing a heavy load, maxing out the rear tires is a good idea since they will have a lot more weight on them than normal. also 3/4 ton trucks run at higher air pressures and of course require LT heavy duty tires that have the proper load index capacity.
do some searching on load index as well, this is the determining factor for tire pressure. but different tires, (P, LT, LR C1 35psi, LR C2 50psi, LR D1 50psi, LR D2 65psi, LR E1 80psi, LR E2 65psi) will have a different load capacity with teh same load index dependent on the load range. its a bit complicated...
how about you just tell us what tires and vehicle you have, and we can tell you the correct air pressure