[mythealias]

it comes down to trust. if i were to guess you lock the door to your house. why would you do something which limits the movement of people who want to be in your house. but at the same time you do not have problem giving access to your house to specific people (people you trust). it is same with countries just more complicated as now you are dealing with bunch of people rather than an individual.

visa free travel exists before countries agreed to trust each other for whatever the reason may be (economic/social/etc.). other countries can workout ways for travel but they do not think of it as a priority.