same reason as this, i suspect:

awk 'END {print 1 * "hello"}' /dev/null
0

the rules aren't really given in the manual page except perhaps
by implication in the BUGS, but in a context where a number is
expected (operand of arithmetic operator) the string is converted
to a number, and "hello" or "foo" are both zero, but "5hello" is 5.
exit takes an expression but doesn't say whether it's integer or
string (but it's integer, as usual on Unix).