history

A Hungarian economist stated that "it was quite obvious that the socialist countries—like other countries—intended to prevent their professionals, trained at the expense of their society, from being used to enrich other countries."[62] Eastern European spokesmen maintained that they were keeping would-be emigrants from suffering from insufficient linguistic and cultural preparation.[63] They also stressed the debt that individuals owed to socialist states, which offered care from birth, including subsidized education and training[63] and, thus, they justified the emigration restrictions as an "education tax" with the states having a right to recoup its investment.[64] Open emigration policies would create a "brain drain", forcing the state to readjust its wage structure at a cost to other economic priorities.[65] Bulgarian and Romanian representatives had long argued that they could not afford to match western salaries and, without emigration restrictions, they "would become like Africa."[65] The restrictions presented a quandary for some Eastern Bloc states that had been more economically advanced and open than the Soviet Union, such that crossing borders seemed more natural—especially between East and West Germany where no prior border existed.

yeah the Wikipedia article basically states what I wrote above

a physical wall only existed in Germany, but the restricted borders were along the entire eastern bloc like @Vampire said, for the same reason