It is theoretically possible to run performance tests with Selenium. You could start up a great big Selenium Grid and then point your grid at an application and run lots of tests against it.
So why don't people normally do this?
The sheer power that would be required to configure a grid that could actually hit your performance testing environment with enough traffic usually makes it a very expensive solution. You then also have the setup and maintenance costs of your grid. That being said, with the advent of cloud services, and tools such as:
It is much cheaper than it used to be. Once you have done the groundwork, it's ...