It works here because the expression is simple. You only have four variables on the stack (maybe more in later models that I'm not aware of) and for those, you are screwed if you just push things on the stack in reading order:

    2 * (3 + (4 + (5 + (6 + 7))

The only way to process this is to determine the most nested expression and start there:

    6 7 + 5 + 4 + 3 + 2 *

... you get the idea.

Expand this expression with multiple deeply nested parens and the limitations of RPN become very obvious.