Why does no one ever come through Earth's Stargate?

Question

It's been a few years since I've seen anything relating to the Stargate, but what I recall from the show is that the episodes go something like this:

1. Stargate team jumps through the gate to another world
2. The inhabitants of that world have a problem
3. ???
4. Entertainment!

All well and good...but that gets me thinking, why doesn't anyone ever come through Earth's Stargate?

The Goa'uld could certainly benefit from a surprise attack in the middle of their base, especially since they have superior technology to Earthlings, and spaceships that could traverse the distance to invade should they render the Stargate inoperable - they have nothing to lose from an attempt.

But even if we don't include the Goa'uld, why don't other species ever cross through the gate in the same way humans constantly hop through to visit other worlds?

Answer 1

Short answer: they try.

Long answer:

They can knock, but it doesn't do them much good.

Without a GDO, the Iris, a large shield millimeters beyond the event horizon that prevents the typical Ka-Woosh (Carter gives a throwaway explanation at one point) and will prevent the reintegration of matter. O'Neil describes it as 'bugs on a windshield'.

In the early episodes of season one, there's an episode where the gate activates off-world, no GDO is used, and a series of bangs is heard. Teal'c and Jackson are watching the gate, discussing what Apophis might be sending through.

Daniel doesn't like the idea that lots of Jaffa just splattered against the Iris, but he likes being invaded a lot less.

The iris is installed while SG-1 is away for the first time (before they even meet Teal'c) and is later replaced by an even stronger version.

Atlantis had a similar thing, but it was a transparent force-field. Unauthorized travelers splattered, but were just flashes of light.

Edit: In response to other species -

Other humans do try to use the gate from time to time. Most use it to travel between a few known addresses (often for trade). Of those who try random addresses, well, they have a very low chance of finding Earth.

It did happen once, in the episode called 'The Other Side'. The people who traveled through the Stargate didn't make it...but the radio call to see if they survived did, and the SGC warned them against sending anyone else through. They established friendly contact, and SG-1 went to visit them. It ended...badly.

Edit: As Matt pointed out in the comments, the Iris prevents the Kawoosh, it doesn't close after it.

Answer 2

First, the is a protective shield, the Iris, as Calccrypto noted in one of the comments. Probably in case of a serious attack that could penetrate the Iris, the Stargate itself could be destroyed, making attacks through it futile (although could be a good way to leave the SG team without the resource of their Stargate).

Second, many of inhabitants of worlds with Stargates seem to be unable or unwilling to use them. So although there's many Stargates, there are not that many travelers or "traffic". Actually, many of the visited worlds seem to host cultures less advanced that Earth's, unaware of other worlds or the possibilities of the Stargate.

Third, I seem to remember from one episode that there is actually a second Stargate on Earth, much older and buried in one of the pole's ice. This could mean that most of those able to use a Stargate lost memories of Earth through huge spans of time.