Why don't satellites hit each other?

This is the best answer on quora ever. A detailed look at the probability of a satellite colliding with debris big enough to ruin it, with great graphs to illustrate the points.  

Here are the TLDR* main points. 

"with $2 billion invested in a launch, you don't want to take even a small chance."

"It is estimated that there are 600,000 pieces of space junk ranging from 1 cm to 10 cm, and on average one satellite is destroyed each year."

"We have three space agencies...tracking satellites and debris. They predict collisions and the affected satellites maneuver out of the way. Despite these maneuvers, we still are losing a satellite to collisions about once every 4 years. By increasing the number of satellites by a factor of ten, unless we start doing better at maneuvering out of the way, we can expect to start losing 2 to 3 satellites a year due to collisions."

"There is about a 0.1% chance of each large satellite colliding with something that will destroy it in a given year."

"Each satellite has about a 0.1% chance of a collision that will destroy it in a given year. That sounds very small. But what happens if you have 2000 satellites? That means on average, you are going to lose one satellite every 6 months, if you don't maneuver them out of each other's way."

"these objects are moving 25 times as fast as a bullet, and all in different directions."

https://www.quora.com/How-do-nations-launch-rockets-without-hitting-satellites-especially-now-that-SpaceX-is-putting-12-000-satellites-into-orbit/answer/Bill-Otto-5?ch=10&oid=150804237&share=2acea928&srid=3um4T&target_type=answer

The exponential increase in debris as successive satellites collide which releases yet more debris, is known as the Kessler syndrome. An uncited line in that wiki entry says it's a risk but not a catastrophic barrier to launching further satellites/rockets. 

"However, even a catastrophic Kessler scenario at low-earth orbit would pose minimal risk for launches continuing past low-earth orbit, or satellites travelling at medium Earth orbit (MEO) or geosynchronous orbit (GEO). The catastrophic scenarios predict an increase in the number of collisions per year, as opposed to a physically impassable barrier to space exploration that occurs in higher orbits" 

https://en.wikipedia.org/wiki/Kessler_syndrome

*too long, didn't read