The reason is because the number of potential ancestors doubles as you trace back every generation whereas the pool of available ancestors shrinks rather dramatically.
After 1670, all bets are off because it was about that time that the colonies became a convenient dumping ground for all sorts of people.
Some were petty thieves and debtors, sort of like the first Australian colonists, selected by the best judges in Britain. Most were just impoverished people.
It was common for a ship to show up off the coast of Ireland or Scotland. Officials would step off dockside and read the names of a list of families who were to report for transportation to America. Generally, they had a day to two to gather with their meager belongings, board the ship and leave. There was little chance of appeal as most of the names had been cleared with the local authorities in advance. Returning from transportation was a hanging offense.
I was talking to my elderly uncle after my father passed away about my dad's temperment. Uncle told me that our family had been booted out of the old country for meaness. I guess there may have been some truth to that statement.