I can only compare it to expensing for purchases that are considered assets. The typical example is that you buy a car for 30k, the car is still mostly worth 30k, so even if you’re out 30k cash you still own a car. So at the end of the year when you’ve made 50k profit and you wonder if you can deduct the 30k from the car, the government will say that the car is still worth a lot of money, so you can deduct 10% of that 30k you paid.
I guess that the government isn’t considering the expense of the employee as much as they are considering the value that the company has retained. Say the developer creates some software that you turn around and license to your customers. That software now has a value, and it will have a value next year too. It probably won’t have a value forever, which is why you can expense a certain percentage of it every year for x number of years.
The money you spent on the employee is probably a direct expense that year, but the product they created still has a value. The simplest way of valuing the product, from the governments perspective, is just to consider it equal to the development cost.