Transplanted Buckeye wrote...
Why can't Excel add numbers in a cell without a leading '=' or '+'.
For
example, 3+2 returns a cell with "3+2" instead of "5".

Turn on Transition Formula Entry, Tools Options, select the
Transition tab in the Options dialog, check the entry for Transition
formula entry, click OK.

Be warned - this will make date entries much more difficult because
1-12-2005 and 1/12/2005 will also be treated as formulas, returning
-2016 and 4.15628E-05, respectively.

If you mean 'Why can't Excel just *add* numbers, but not subtract,
multiply or divide them without a leading = or +?', then I suppose it's
because Excel's developers believe consistency in this instance is more
important than convenience only for those who want to add.