[Someone]

It doesn't really _need_ it; the seat stay could be designed to be stronger, but there's more of your weight on the back wheel than on the front wheel, so the seat stay would have to be stronger and, hence, 'fatter' than the fork.

And, since the angle between vertical and the seat stay tends to be larger than that between vertical and front fork, the seat stay already would need to be a bit fatter (the fork is relatively more compressed and relatively less bent than the seat stay)

Also, the force you exert on the chain when riding the bike tries to bend the seat stay, too. That, too, would require a stronger seat stay.

However, there is a better (in the sense that it makes the frame stronger at the same weight or gives you a lighter frame of the same strength): the chain stay. It introduces a triangle in the structure, and triangles are strong.