Why can't we use helping verbs in positive sentences of present simple tense and past simple tense? 

Such as:

> 
> She makes tea.  
> 
>  They went to the park. 
> 
> 
> 

If we convert them into negative or interrogative sentences, we use helping verb; but in case of other tenses we use helping verb in all formats of sentences i.e. positive, negative, and interrogative.