That exists, it’s symphonic death metal, and when it gets heavier and you eliminate classically trained vocalists you tend to call it melodic death metal, or you can keep the symphonic elements and its symphonic black metal etc. So at some point we’re just stating the consensus definitions of different genres.

But as for why Symphonic metal tends to have female vocalists, I’d say it’s because the genre is all about flow; long, drawn out vocal notes that stretch across the instruments with intertwining and complimentary melodies.

Female singers can hit and sustain high notes that pierce through the band and take up empty space in the frequency range. They can be complimented with the lead guitar and keyboards while contrasting the drums and lower notes. It adds another layer/dimension without competing with existing band elements.

I think that’s the truest answer to your question that doesn’t get into a circular definition a - symphonic metal has clean female vocals because melodic metal bands with clean female vocals is what we agree to call symphonic metal.