In cities, I agree that if you aren't used to it, it might be inconvenient.

However, 99% of hikers in Europe carry their own trash bags. It took off in the early 2000 and now it's almost a culture thing, especially amongst young hikers, and not having one is basically a social error (well, littering is, you can always ask people for an empty trash bag if you forgot yours). Honestly I can see why they choose that solution.