Practical tests and empirical data have shown that deformed bars (rebar) hold very well in concrete to the point that the concrete or rebar itself fractures and breaks off before movement occurs. The point of the overlap is to prevent a weak spot along the member. Overlap is usually defined as 36–48 diamaters of the rebar in question. This gives a large factor of safety in the design.
Welding requires additional equipment and personal present to perform. Typically, this is done due to gap requirements (allowing aggregate to pass between rebar pieces), special coatings on the rebar, or deployment in case of nested cages. I have come across a few industries that choose to always weld, but that usually ends up being for other considerations such as the amount of rebar used in a project.
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