Here's why:
* Old English (also known as Anglo-Saxon) was the language spoken in England from roughly the 5th to the 12th centuries. It was significantly different from modern English, both in grammar and vocabulary. Think "Beowulf" and the "Anglo-Saxon Chronicle."
* American English is a variety of Modern English, which developed from Middle English (around the 12th to 15th centuries). Middle English evolved from Old English due to the influence of Norman French after the Norman Conquest of 1066.
Therefore, American English is a descendant of Middle English, not Old English. While Old English has some influence on modern English, it's not a direct ancestor of American English.