Because the rate of unfounded claims (false claims and/or claims for which supporting evidence cannot been found) is about 8%. Which means approx. 92% of the time, there is at minimum some truth to the claim and evidence which supports it, but it won't be found if the police disbelieve and dismiss the person making the report.
Interpreting the effort to get police to not ignore women reporting rapes as "don't bother thinking, just assume everything you're told it true" is ridiculous.
Nobody's saying not to investigate. The problem is people who say the victim should
automatically be believed--that is, don't investigate.
And note that that 8% number is FBI stats for cases
proven false. It doesn't count the cases which can't be disproven and it doesn't count the cases where the police realize it's fake (usually someone trying to cover up misbehavior) and dissuade the woman from making a report in the first place.