Here's why:

* Acids are proton donors: They release hydrogen ions (H+) into solution.

* Acids do not react with other acids to form new substances: They simply coexist in the solution, increasing the overall concentration of hydrogen ions.

Example:

If you mix hydrochloric acid (HCl) and sulfuric acid (H2SO4), they will both contribute hydrogen ions to the solution. The solution will become more acidic, but no new compounds will be formed.

Important note:

While acids don't react with each other, they can react with bases to form salts and water. This is a neutralization reaction.