Why do you believe Jefferson chose to include the list of grievances in the document rather than summing them up with a single blanket statement?

Setting down a litany of woes like this is so much more effective than just putting all the colonists' grievances into a couple of sentences. Listing all the grievances separately adds a suitably condemnatory tone to the Declaration of Independence; it's as if a serial criminal is being confronted with the damaging effects of his lengthy rap sheet.
The Declaration was a rhetorical document more than anything else; it was designed to convince those still unsure about the wisdom of breaking free from the British that this was the right thing to do. So Jefferson, in listing the grievances separately, rams home the point more effectively that the British have consistently ignored the legitimate concerns of the Americans and that independence is now the only way forward. Jefferson was also doubtless aware that adopting this rhetorical strategy would make it easier for each individual grievance to form the basis of anti-British propaganda in the ensuing struggle.

In the Declaration of Independence, Jefferson was trying to make the rational argument for American self-rule as well as convince those who were undecided on the issue that the time for independence was now. By listing the grievances individually, Jefferson hopes to incite feelings of anger in those colonists who may not have made up their minds. In 1776, not all colonists felt as though separation was the proper way to deal with Parliament. Many feared American self-government—the new country would be vulnerable to attack and no one was sure how it would be governed. When Jefferson listed the problems faced by the colonists, such as trials without juries and unreasonable tax collection methods, he wished to make the colonists feel as though separation would be better than just staying under British rule.