There's evidence of soap right back to 2800 B.C. Originally ashes and animal fats were combined and it was commonly used for treating skin diseases and for washing. There's a fantastic article here about the history of soap.
Much of the concern surrounding bar soap is the idea of bacteria living on the surface. Studies show that this needn't be a concern - even if bacteria is present on bar soap the evidence shows that transmission is highly unlikely - this article sums it up. Liquid pump dispensers can also harbour bacteria, though as long as you wash your hands thoroughly and don't touch the dispenser again, the soap will do it's job to wash the bacteria away.
Generally speaking we use seven times more liquid soap than bar soap... whether this is because we feel more soap means better cleaning or that pump dispensers are overly generous, the fact that we use more remains.
Many liquid soaps tend to use synthetic detergents which can dry out the skin and as they contain water are generally heavier which causes concerns with shipping.
Soap bars can also be packaged with easily compostable packaging such as paper or cardboard or even left without packaging at all.