The Jews avoided the word "God". So Jesus would not say "I am God" - it would appear a blasphemy inside a blasphemy.
Instead, he choose expressions of even well-known formulas to mean the same thing.
As the passages cited by Finrod show, the jews got the message, so much they tried to stone Him.
Another example:
Jo 14,6 "Jesus saith to him: I am the way, and the truth, and the life. No man cometh to the Father, but by me."
Imagine a leader saying "I am" those things, not "I tell about", "I know about" or "I received a message from God about"
or he is god, or crazy