English translation:
there is no need to
never happens
Structure:
Verb + ことはない
Any particular reason why it’s ことがある for “it happens” and ことはない for "it never happens?
Just the nature of the Grammar Points. The fact that the translations have this polar-end opposite meaning to them is just a coincidence. If you were to say, ことはある or ことがない, depending on the sentence they could work, but remember that these are Grammar Points for a reason.
Hope this helps!