I think one of the reasons why rock covers bands like female singers is that once blokes advance in years, in general, their high pitching goes and most rock songs are sung quite high, as such females can generally reach notes that males left behind years ago, assuming they could ever get them in the first place.
We had a similar conversation with the guy at our rehearsal studio, he said given the material we do - Rainbow, Deep Purple, Led Zep for example - we`d be hard pushed to ever find a male singer, other than one in their 20s, who could do that stuff and realistically would said guy that was able to sing that material be satisfied with being in a band with blokes in mid/late 50s, rather than put his talents elsewhere.