?
Because gravity sucks.
::Applause::
Spheres are the least surface area to enclose a given
volume.
In a solid spehere, the average distance from a given point
to the center of mass is minimized.
When a heavenly body reaches a certain mass bup’s explaination takes hold and the body becomes round. That is why our moon is round and Mars two moons are not round.
If the world didn’t suck, we’d all fall off.
In general, planets aren’t spherical. Consider the element of obliquity.
I prefer the element of ubiquity myself.
Are you using “obliquity” in the sense of “Obscurity in conduct or verbal expression”?
Wanna know why?
Because a flat surface has a greater gravitational pull than a rounded one(like a magnifying glass, kind of, except the opposite). All of the “stuff”(space debris. etc.) gets pulled onto those flat surfaces, but not necessarily in the portions needed(sometimes more sometimes less) to make the earth perfectly round. So after the debris has been magnetically attracted to those flatter parts, they become rounded and the outskirts of the areas once flat become flat too, and the rounding process continues, over and over again. 
I thought the Earth was very slightly pear shaped?
Tha’ cho mama foo!
Nope: the Southern hemisphere is supposed to be slightly larger than the North.
Maybe gravity is pulling it down 
The earth is pear shape only in the sense a watermelon could be considered a cube. No, not even that close.
When we talk about the shape of the earth, we are talking about its gravity field–the shape of the water surface if all the continents were criss-crossed with canals, essentially. That shape is called the geoid, and it is that shape that influences the orbits of satellites. By analyzing the orbits, we can determine the shape.
To first approximation, the earth is a sphere. Even its twenty kilometer bulge at the equator, due to rotation, would allow it to pass muster as a billiard ball on some tables. That is hardly pear-shaped.
The centrifugal bulge is considered a second-degree shape. Imagine a cross-section going through the poles–a low at one pole, high at the equator, low at the other pole, and high again at the equator. That’s like two periods of a sine wave, hence degree-two. If you do the same thing with three periods, you get the degree three pear-shape. The earliest satellite analysis determined that there was some slight effect from such a degree-three gravity field, and that was known as the pear-shape.
Subsequent satellite analysis has shown that other degree three shapes (there are six others) have even much more power. They all resolve into a single degree-three shape which is most accurately described as a rounded-off tetrahedron, which is not a pear-shape but could be called one if you squinted–and its four points are at most 100 meters, which is swamped by the twenty kilometers of the degree two. Also, one of the points would be on the equator.
As to the OP, another way to answer it is that the gravity field of any mass tends to be spherical–the farther away, the more spherical. That is equivalent to the fact that mass can be treated as a single point at its center of mass, to first approximation. Then, matter tends to form equipotential surfaces, which is another way of saying “water flows down hill”. What you end up with, after the matter has “flowed”, is a pretty good sphere, for large bodies.
Well, bumpus and RM Mentock are correct about how gravity makes things round, but there are two forces operating towards that end: gravity and surface tension.
Gravity, the force pulling the surface inward has been exhaustively explained.
Surface tension is a sideways force that pulls the surface together, seeking to minimize the surface area. It’s why when you blow soap bubbles, they’re round. Or when an astronaut plays with his tang, it floats around in spheres. [sub]God I just reread that sentance in preview, and it sounds like a dirty joke![/sub]
But wait. Planets aren’t soap bubbles or blobs of orange juice! Well, no, but they were once close. During their formation, they were hot and molten, and surface tension and gravity worked together to shape them into spheres.
I don’t know about that. Once you get up to planetesimal size, doesn’t gravity pretty much overwhelm surface tension? I could be wrong. What are the numbers here?
If you throw a tang bauble into the air, it pretty much splats, on earth. The difference between the size of the sphere, and the size of the puddle, represents the difference between the strength of gravity and the strength of surface tension. Even for a three-inch water balloon, when it breaks, it’s going to create a pretty flat wet spot. (you’re right, this thread should be in the cecilingus forum).