Why not ai is written a in french language

In french language you have to pronounce 

Ai as A (English pronounce)

Au as A  (English pronounce)

Ou as Oo  (English pronounce)

Ea as A

Oi as Wa (pronounce "was" keeping s in silence)

Why the pronunciation is one but you have to write two or more letters. 

One guy told me that, long long ago, a King was paying money to writers based on how much letters are used to pronounce any word.

Means a word must be pronounced in less or same tone but letters in words should be many. 

Using this trick writers were earning more and more money from the king. 

This is the reason why we find in french language what it is pronounced that is different in writing.

Many letters are not pronounced or combinations of letters make completely different pronunciation.

If you don't know french language, then it is difficult to write a word just by hearing the pronounced letters. 

In English and Spanish it doesn't happen. 

Any one can write a word just by hearing the pronunciation of that word of English or Spanish, off course, pronunciation of each letter of that word.