Why Do Drivers Do This?

I was driving home tonight on 50th Street, which is an east-west street with a speed limit of 40 MPH.  This is a 7 lane street: 3 east bound lanes, 3 west bound lanes, and one turning lane in the center.

I was driving west in the left most lane. I was traveling about 38 to 40 MPH.  The center lane was vacant, and the east bound lane had a vehicle in it, a little behind me.  From the right, a large SUV pulled into traffic, crossed two lanes and proceeded in the left most lane, in front of me.  And was traveling at less than 30 MPH.  After a few blocks, I swerved into the center lane, passed this vehicle, and returned to the left most lane, as I was needing to make a left turn in a few blocks.

I looked back at the driver. He was an older person. He was eating, talking and looking around while driving.  I wonder how much of that influenced his driving style.  I wondered why he had to be in front of me.  I increased my speed to close to the speed limit and soon was well in front of this vehicle and its inattentive driver.

Why do drivers do this?