Why does rockets send from Earth to other planets only travel with 11 km/s? Why not faster?

11.2km/s is the escape velocity from the earth's gravity. This means that in order to lift up off of the earth, this speed needs to be reached. Believe this was the speed used by the Apollo launches, although this certainly doesn't mean they CAN'T go faster, they simply didn't need to go faster to get to the moon.

Our technology is not capable of moving solid structures to other planets except by giving them a shove and then letting them coast. Even if we spend the enormous amount of money needed to get fuel into space to boost the speed of the rocked to double what it is now, the distances are so great that the rocket would still be coasting almost all of the way. So there would be little benefit to the expense.

Look at it this way: If you were the head of a government agency with a limited amount of money available for space exploration, would you rather use the money to send one rocket to Saturn and get your answers in four years, or would it be better, for the same amount of money, to send four rockets, one to Saturn, one to Uranus, one to Jupiter and one to Neptune, all getting there in eight years from today?

When we have some method of travel other than coasting, then we will increase the speed.

Because they don't need to go any faster. Going faster costs extra fuel, which means extra mass and extra money. *Lots* of extra money.

Many space probes use gravity assist in their journeys, and end up going lots faster than 11 km/s. This requires some care if they want to go in to orbit when they get there, because they must have some way of slowing down.

The New Horizons spacecraft, launched 2 years ago, was the fastest spacecraft launched from earth to date, reaching a maximum of 16.2 km/s relative to the earth. This craft is scheduled to arrive at Pluto in 2015. Don't forget that the earth's speed in its orbit around the sun is 30 km/s, so all spacecraft leaving the earth are given a boost by this amount.
If a mission can be accomplished within a sensible design lifetime of a spacecraft, there's no point in wasting fuel to boost its speed only to have to slow the spacecraft down again when it reaches its target.

11.2 km/s is indeed the SURFACE escape velocity for the Earth. What this means is that at the Earth's surface, this is the escape velocity. Remember, that as we get further and further from the Earth, escape velocity decreases, since gravity decreases with distance.

Anyhow, spacecraft traveling from Earth routinely travel MUCH faster than 11.2 km/s. For example, the Voyager 1 spacecraft is currently traveling at a speed of 17.374 km/s, relative to the sun, and Voyager 2 is traveling at 15.957 km/s, relative to the sun.