Question

why sulphuric acid does'nt have pH?

Answer 1

this is wrong.
ph of sulfuric acid is 1.5

Answer 2

Strictly speaking, it does, because it is the negative log of the hydrogen ion concentration. However, for strong acids you need a rather massive dilution to persuade yourself there is a major difference. Ideally, you would have to dilute by a factor of ten to move it 1 pH unit, BUT there are further complicating factors. For strong acids, like hydrochloric, the "concentrated" acid in water has a pH in the order of -4.5. If you dilute that there is a good reduction in acidity, but you need some means of measuring it in these negative regions, and it is not linear with water. Part of the problem here is what is called activity coefficients, which reduce the effective concentration as concentration increases, and other coordination, which is a way is one of the causes of the lower activity coefficients. You do not see hydrogen ions. In water, the hydrogen ions are protonated water molecules, and a lot of water clusters around the protonated ions. Think ammonia - the ion in solution is the ammonium ion. There are no totally inert solvents that dissolve strong acids, as far as I know. Concentrated sulphuric acid, for example, has a very strong acidity, but it is still not due to protons, but probably due to some cluster around H3SO4+. If you dilute that with water, you get a dramatic reduction in pH, although pH in those regions may be thought to have a different effect. Thus fluorosulphonic acid behaves even stronger than concentrated sulphuric, presumably because the fluorine better hydrogen bonds with the hydrogen ion and the forms a more stable entity.

Weak acids  have a different effect. If their dissociation constant is such that there is only weak formation of H3O+, then you have the main component in concentrated solution as the undissociated weak acid.  As you add water, a little more of the weak acid dissociates, and the pH remains more or less the same.