Besides culture, I think money is the main reason. How much money would these girls earn every month working a regular, 9-5 job? If that is even available?
Let's take a hypothetical Eurobabe i.e from Czech, Hungary, Romania, Russia and Ukraine. Czechs have the highest per capita income per year at $18,000 and Ukraine the lowest at $4000. But how many Czechs earn at least this amount ($18,000). Seems like 18-early 20s girls in these countries without much education don't have a lot of options.
That's the reason you see many eastern European girls working as models in Asian countries (Japan, Thailand, India) where the western look is sought after. And these are real models, not escorts or pornbabes.