The main reason why this works is that Berlin is still really cheap compared to London but even compared to Hamburg or Munich. Rents are getting up though, still the amount of great food and entertainment for relatively cheap makes it easy to live with that salary.
I read an article from Buffer where they equaled 60k in Berlin to about 120k in SF or NY. I also think salaries in London are not substantially higher, while the city is so much more expensive.
I am still looking to go contracting at some point to make more money to raise a family, but for now while sharing an apartment with my girlfriend, it's decent money. Most of Berlin's working population gets far lower wages anyway, even in startups outside of development so for me as someone without an degree (yet), it always feels weird to earn substantially more than people that have a Masters in Business/Marketing.