https://en.wikipedia.org/wiki/Passport_system_in_the_Soviet_...
/s
There was multitude of reasons but the main ones were what you needed to have a good standing in the society[0] and the need to control every aspect of that society. People who would see a (even a little) better living could start asking questions. One could say that was a result of fear.
[0] the article provides one such example:
> Because of Dmitry’s active involvement in the communist youth groups, when he was 15, his school sent him to the Pioneer summer camp in Nordhausen near Erfurt in East Germany.
Also: https://www.indexmundi.com/facts/indicators/ST.INT.DPRT/rank...
The European numbers are probably a bit inflated due to Schengen and their closeness.
In 2019 Germans made more international tourism trips than americans, despite having only a quarter of the population.
But the observations depend on where you travel and see other travellers, right? Go to nw mexico and I expect most tourists are from the us? Whereas mallorca is practically the southernmost german bundesland? And cyprus presents plenty of russians?
And comparing geography: Germany is geographically much smaller than the US, and in a place like Liechtenstein it is easy to make an international trip just to go shopping, even if you take the bike.
Also factor in that many people who do fly, fly multiple times a year.
If that 93m is individual passengers then you should discount it quite a bit to accommodate frequent fliers and overseas tourists returning home.