It solves different problems. flair doesn't include parsing, Spacy doesn't support embeddings, gensim doesn't do tagging or parsing at all but contains the most practical word2vec implementation. NLTK is nice for learning, but don't use it in production unless you're ready to reimplement things in a more efficient way when parts start falling off.

The message is - again - learn about the mental framework, not individual tools, to understand where each strengths are and what the gaps are in between them. Or choose a problem, find the best tool for that problem and get progressively better at the tool(s) that helps you with most of your problems.