Earlier quoted context omitted.
Even in modern times, international air tickets are more expensive and worldwide wages lower than you imagine.
Perhaps, but there are other options -- international bus tickets are pretty cheap. So is the ferry between, for example, Morocco and Spain. Plenty of people could afford this, or have friends/relatives in wealthy countries who could pay for them.
Passport control as immigration regulation/deterrent definitely rings true to me, and some countries see less need for it with richer travelers.