Why must G-d emphasize this so much in the Torah?
In the Gemara, there is a story about a big Rabbi who was learning once, and a "spirit" came to him. This person was a Mumar (person that violated Shabbat in public, or worshiped false gods in public). The Rabbi asked him "how could you turn from G-d, when he did ALL those amazing miracles right in front of your face?"the man answered "if you were alive in this generation, you would run to kiss the gods' feet".
This teaches us not to be so judgmental of others who do bad things, and always be "Dan Lechaf Zechut- judge favorably".
We must not judge others, and always remember Hash-m is the judge not us!