Some writers deliberately write in a way that makes their text difficult to read.
They do this to create an air of mystique or superiority or to appear more ‘educated’ than their readers. They emphasise the supposed distance between their intelligence and that of the reader by using words that their readers may not know and putting it altogether in long and complex sentences.
This is a pretence! It always gets in the way of good, effective communication.
If you feel you need to impress you readers in some way, do it with the power and clarity of your thinking, not by showing off with your language or vocabulary skills.
And sometimes, difficult writing is simply a cover-up for a lack of clear and insightful thinking. Some writers use complex writing in an attempt to sound profound when they are not really saying anything worthwhile at all.