Mostly for simplicity. Most folks can quickly tell you their annual salary, it would likely be more difficult to calculate their post-tax income. There is also the possibility that individuals may prefer to withhold more or less throughout the year depending on how they approach taxes, so that could potentially skew the numbers.
Ah alright. I pay nearly 50% taxes here in germany and it gets deducted from my loan before I get it, so it's probably a bigger deal here then elsewhere.
