It's because that's the way English verbs are conjugated. English actually has a very simple way of conjugating (regular) verbs. Most of the complexity of the ancestral Germanic has been lost. Contrast that to other Germanic languages. Swedish present tense, for example, is very simple. German still retains a complex morphology.
The only inflection English retains in the present indicative is the -s suffix for 3rd person singular (frog). Third person plural (frogs) does not have an -s.
The -s suffix for noun plurals, and for possessives (vestigial genitive case), add exceptions that make learning English a little harder.
The only inflection English retains in the present indicative is the -s suffix for 3rd person singular (frog). Third person plural (frogs) does not have an -s.
The -s suffix for noun plurals, and for possessives (vestigial genitive case), add exceptions that make learning English a little harder.
Edit: grammar :)