Because the whole gram per watt nonsense is bullshit....
Thats not not to say its impossible, it's just nonsense to think that unless you're raking in over a gram per watt that your grow is wasteful and inefficient.
the ounce per foot rule is more realistic and far more average of what most folks get out of their grows, it also helps knowing how much light you need to fill a space.
Gram per watt rule is for specific strains that have the capability to put out a gram per watt.