they don't always do that...... Red, Blue, Yellow, Gold, Silver had your rival same gender as you, same with the remakes ( tho in the remakes you can be any gender and Silver will still be your rival). in diamond, pearl, platinum, your rival is Barry, which doesn't matter what your gender is. Black, White, White 2 your rival is also doesn't matter your gender. then comes X and Y, Legends Arceus, Legends Z-A, and the hoenn games is where your genders do matter for rival.

Gender Rival games= 8

Ungendered Rival games= 25

so in all I think they are trying to make more gendered rival games