Question:
Why don't we do a "Sholom Nekeiva" (lit. welcome female) upon the birth of a girl, as we do a Sholom Zochor for a baby boy?
Answer:
A girl doesn't need a Sholom Zochor for the same reason that she doesn't need a Brit.
Male and female souls are different in their makeup, and come from different sources.
For the male soul, physical and spiritual are two opposites in conflict - you can only have one or the other. For the female soul, physicality is just as holy as spirituality; they are in harmony and can coexist. This is because the male soul is sourced in G-d's light, G-d's revealed self, which shines in the spiritual world, but is concealed in the physical world. The female soul comes from G-d's essence, which is everywhere equally.
So, the male soul's mission is to aggressively conquer the physical world, to bring G-d's light there. That's why the six days of the week are associated with masculine energy, and the Shabbat with feminine energy (the Shabbat Queen, the Shabbat Bride). For six days, we work to aggressively change the world, a male pursuit. On the seventh day, we appreciate the innate beauty of the world - a feminine attitude.
The Brit symbolizes the male mission - the power to take the most physical object and transform it by cutting away the external layers that conceal the inner light. A female soul doesn't see the need to cut anything away; there is holiness within the physical as well, it just needs to be nurtured, appreciated, and recognized. So, she doesn't need a Brit. And she doesn't need to be consoled when she is born - she intuitively understands the potential this world has for holiness. For the male, birth is a steep descent, which needs to be justified. For the female, it is not a descent in the first place; she needs no explanation.