To me, it seems like such a waste of energy to teach children a new set of symbols $(0,1,2.....)$ after they have learnt the alphabet. Why do we not replace $a=0$ and $b=2$ until $j=9$ and use the other letters as operations such as multiplication and division and use the upper cases for $A=a^{2}=0^{2}$ and $B=b^{2}=1^{2}$ and so on. I believe that most people are more fluent with the alphabet than the numbers, hence why you read so fast. Also, I do not think it would add to confusion if it is a sentence or an equation since the chances are so low and it should be clear depending on the back ground. Would this not allow for efficiency and make it easier for people to grasp math (at least at school level) ? Are there any problems with this way? I have been thinking about it and have not found any big disadvantages.
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TL;DR: That's just the system we chose.
Slightly Longer Version:
The idea of using numbers as letters exists in some languages: Hebrew, Greek, Coptic, etc. used letters as the basis of their numerical system.
The problems arise however, when we want to use variables, or use numbers in a sentence. For instance:
"O men!"
Could mean "15 men!" or as an interjection like here.
Variable declariations would also be a problem: the equation $ x = y + 2 $ could evaluate to $ 24 = 25 + 2 $ or as the linear equation we all know and love.
All in all, that method of numerical systems is outdated and problematic, which is why we adopted the Arabic numeral system.