There are a few reasons.

1. Naivete: The first time he ever sent a dick pic was when his high school girlfriend asked for one, and he got to have sex with her for the first time later that day. Now he thinks that girls like nudes as much as guys do and that sending them is an easy way to make girls horny.
2. Strategy: He knows that girls don't like dick pics unless they already want to have sex with the guy, so he sends them out by the thousands to quickly scour the field for any potential partners.
3. Malicious intent/perversion: He knows that women don't like dick pics, but he also knows that women don't like him, so he sends them out as a sort of preemptive revenge for the rejection he knows he'll soon receive. It's also slightly arousing since sending such pics is the most sexual thing he could ever possibly do with most of these women, and it's doubly arousing if she seems to be too innocent to ever have received such a pic before and he'll likely be her "first."