The answer to this question is pretty simple. Why? Because it is spoken so fast! Native speakers of any language speak their own tongue very, very quickly. They run words together, speak with different dialects, different accents. They use slang phrases, curse words, metaphors, cultural and local references that the non-native speaker may know nothing about. Try to compare the English spoken by a New Yorker, to the English spoken by an American Southerner to English spoken by a Scots. Now, that's a lot of different English!
The only real way to get 'on top' of this phenonmenon is by listening to it spoken again and again.
For instance, I love British TV mysteries and watch a lot of them. I am pretty sure what they mean when they talk about the 'telly', the 'jumper' the 'torch' and 'getting it sorted'. I have been watching Outlander this season and can almost understand the Scottish brogue, most of the time!
So, in the end, practice listening and then, some more practice. Find a TV show you like to watch and listen to them!. Courtney W.