Well, you have Scotland Yard to deal with the big boys. Greta's is more you're local bobby on a bicycle. She can't be expected to tackle the global superpowers, but by Christ they know about her.
The hope, I imagine, is that if enough (comparatively) smaller nations take affirmative action in tackling climate change, that the bigger ones (well, not China. They're beyond hope) might take notice, and perhaps be shamed into following suit. She has addressed the UN in the past, so she isn't solely picking on the U.K.