I know that big websites suffer from scalability issues and I guess that more servers (hardware) can help to handle scalabililty issues but I see that big websites are stuck about scalability even if they can handle the cost of new servers.
So why can big companies not handle scalability issues by more servers?
Best Answer
By introducing more servers, you are introducing more lines of communication and the need to keep things synchronized. This is not trivial.
The amount of communications overhead goes up quadratically with the number of nodes communicating. If you try to centralize communications you then introduce a scalability bottleneck with communications.
There are several known architectures used for scaling and none of them are simple as "add more servers".
The High Scalability Blog have featured a recent blog post just this subject.