I guess I don't understand this sentence, particularly why 'là' is there. Could 'là' be omitted? Google seems to say 'yes'.
heard as "Nó không dễ như bạn nghĩ" does not have là
Can any native speakers confirm whether or not this is a common saying?
Yes, and both "nó" and "bạn" can (and usually) be replaced with other words.
Why can't this be "không đễ như bạn nghĩ' as the English "it" seems often dropped at the beginning of the Vietnamese equivalent. For instance:
"không phải cố tình" -- "It is not intentional"