You have just underlined the problem of cricket. In baseball it is relatively easy to prepare accurate statistics for each fielder:

- The field is smaller
- Each fielder has his own zone to defend, which is well defined and has its own boundaries.
- There can be defensive shifts which can move fielders from their original positions, but these are not "extreme". The manager can ask his fielders to play closer or further from the plate, more to the left or more to the right, depending on the batsman they have to face. However, the zone covered by the shortstop will always be the same. And the same goes for each other fielder.
- Limited number of batted balls. How many balls are hit in fair territory per game? I don't think more than 50. Even less, if we exclude HRs.

In cricket, this is much more complicated
- The field is bigger.
- If the captain wants an attacking field, he will ask his fielder to be in a specific position, which is COMPLETELY DIFFERENT from a defensive field. There are so many options for a captain and can be easily noticed even by non-experts. Sometimes defensive shifts in baseball can only be noticed by experts.
- Greater amount of batted balls.

It will take much more time (even in baseball, fielding statistics are far from being perfect), but I think it is possible to find a good method to evaluate fielding in cricket.