Why is the frequence electromagnetic radiation measured in wave secound or hertz?

Answer 1

Because that's how frequency is DEFINED.

The frequency of any wave, including an EM one, can be described as the number

of times per second that wave repeats an action. Thus, crests per second, or Hz.

Just think about the word "frequency". It means "how frequent" or "how often".

To describe how often something happens, you count how many times it happens

in a week, a day, an hour, a minute, or ... if it's something that repeats very fast ...

how many times it happens every second.

That's exactly the meaning of the unit "Hertz" ... it means "per second".