The thing is...
1) Fielding standards are much, much higher now than they were back in Viv's time.
2) Sachin plays SO much more cricket, yet his body is able to handle it and he's been delivering the goods for so long. It's non-stop cricket, the fact that he can maintain this record despite barely having a break is incredible.
3) Technology has come to a point where bowlers can analyse everything a batsman does before a match, even down to the smallest details. The smallest movements, flaws, weaknesses. Sachin has been under scrutiny and studied by bowlers for all these years, 2 decades, and they have yet to find a way to contain him.
4) Sachin opens the batting, ie new ball in swinging conditions. Viv batted in the middle order. So they had two different roles - Sachin isn't meant to score at a strike rate of 90, his job is usually to bat through the innings while Viv's was to demolish the opposition bowling. Sachin has shown himself capable of tearing bowling attacks apart when needed to, but hasn't done it that often.
Now, what plays in Viv's favour were the better pitches for bowling, no fielding restrictions (which Sachin enjoys as an opener), and he never wore a helmet.
But the incredible fitness and longevity of Sachin, and his ability to keep churning out runs despite all the travelling, hectic schedules, media attention, scrutiny and planning and studying from the opposing team is just incredible. Almost super-human. And that's why I pick him.