Actually, we do have some dimensional lumber.
But let's discuss hardwood lumber and why it's random in size.
The hardwood industry aims to serve furniture makers, cabinetmakers, artists, and other types of custom craftsmen. And because of that, their projects are frequently custom -- that means there are no standard sizes that will work for everybody. So, selling wood that's been processed down into uniform sizes only adds needless labor, machining, waste material and therefore cost.
Random sizes are also a matter of practicality. Trees are roundish and random in size, too. So the lumber each tree yields is random.
Food For Thought
Why pay the extra cost for pre-cut dimensional sizes if you're just going to cut down to whatever your project needs anyway? Random will do, and random will cost you less.