Question

Why we don't do it? ​

Answer 1

You got a interesting question;)

So here's your answer

Lets assume we have to prove a=b

According to your logic we can multiply both side by 0, but you can't do it because after multiplying both sides by 0.You are proving

$a \times 0 = b \times 0$

not a=b.

Hope this helps^_^