English spelling is phonetic, to a certain extent. It's just it was meant to (more or less) write down the language as it was about 700 years ago, and there's not been a consistent or generalized effort to update the spelling to reflect the newer developments in pronunciation.

Why is English so irregular?

The English spelling system developed over the centuries and the irregularities came about because of various invaders and writers trying to fit their alphabet and sounds to English: English developed from the Anglo-Saxons & Vikings from northern Germany and Scandinavia.

Is English an inconsistent language?

The peculiarity of the English language phonology is that it is highly inconsistent. These inconsistencies are widely spread in the areas of differences in the spelling and sound representation, sameness in sound or letters but different representations.