They don't. There are three phases in Tendulkar's career:
1. From debut to the end of the 2003/04 season before his Tennis elbow injury. Excluding Bangladesh he averages
57.63 over
14.5 years.
(There is no justification to remove runs against Zim prior to the player exodus in 2004 since the average of batsmen against Zim in this period is much lower than that against say WI in the last 7 or so years. But if you do remove them he still averages 56.14)
2. During his tennis elbow injury when he tried to continue playing without getting the required corrective surgery. This was a period of
3 years when he averaged
31.51 excluding Zim + Ban.
3. Since recovering from surgery to today: A period of
3.5 years when excluding Zim + Ban he averages
59.88.
So overall apart from the 3 years of injury, he scored
12500 runs over
18 years at an astounding average of
58.17. This is excluding Ban and Zim post 2004.