You can't just "leave them there" though. They orbit at high speed, which effectively means they actually take up vastly more space, with other objects orbiting at high speed intersecting those orbits. The orbits that are most useful are relatively narrow bands shared with a lot of other satellites and a fair amount of debris, and orbits tend to decay over time (which is a problem if you're in low earth orbit because they'll decay all the way into the atmosphere, and a problem if you're in geostationary orbit because you'll lose the advantage of stationary bit for maintaining comms links). This is a solvable problem with propulsion, but that entails bringing the propellant with you and end-of-life (or an expensive refuelling operation) when it runs out. The cost of maintaining real estate space is vastly more than out right owning land.

Similarly, making stuff have a great life expectancy is much more expensive than having it optimized for cost and operational requirements instead but stored somewhere you can replace individual components as and when they fail, and it's also much easier to maximise life expectancy somewhere bombarded by considerably less radiation.