Because math is abstract. The variables and constants don’t mean anything in most cases and are only given different symbols to distinguish one from another. Descriptive variable names or even descriptive subscripts get OLD very quickly when you’re working through a page of calculations to solve a problem.

The beauty of math, if you’re using it to solve some real world problem, is that you can forget about what the symbols mean and focus on solving the equations, evaluating the integral or derivative, or whatever other calculation you’re doing.

Once you’ve completed all of the calculations and gotten your answer simplified as much as possible, then you can go back to the problem you were trying to solve and interpret your result in context. Since you’ve done all of your calculations purely symbolically, you can see the relationships between the quantities you’re working with. In many cases, some quantities you thought were important actually cancel out and don’t appear in your final answer at all. This tells you something fundamental about the independence of your quantities. Perhaps the most famous case of this comes from physics: the acceleration of an object in free fall is independent of its mass. You would not see this so clearly if you substituted numerical quantities as soon as possible.