It's not talked about very frequently because it is a very fluid entity and cannot be quantified as precisely as Cost Per Mile and Cost Per Week. Fixed costs like insurance, and operating costs like fuel, can all be determined with relative certainty and accuracy.
On the other hand, Profit Per Mile and Profit Per <timeframe> are solely dependent on market conditions, provided your expenses are under control. If you know what your expenses (and a few other mitigating factors, like where the load is taking you, etc.), then you will know what the minimum amount is for which you can take a load and remain profitable.
Before someone could say, for example, they must make 35% profit on a given day, week or month, all the other financial factors would first have to be determined. And if you first determine those other factors, Profit Per <timeframe> literally takes care of itself. Profit Per <timeframe> isn't something of which a goal could be constructed (for anyone who isn't wholly anal), since it's wholly dependent on everything else. It is an end result score, a "look what we did" kind of thing, and not a "we need to achieve this" type of thing. Just like any other business, at the end of a given timeframe, you look at your profit, and if it's not where you want it to be, you look at places where you can adjust either your expenses, or increase your income. That's where you get to the "we need to achieve this" stage. But you don't start off with a profit margin percentage to begin with, particularly since it will be different for every driver, more different than the CPM would be.