Can someone explain this to me in idiot english?

I don't understand why would exhaust parts cause a drop in MPG?

 

As 02 sensors get old, they cycle less and less. Allowing periods of longer rich a/f before they can respond. also old 02 sensors will tend to start reading the wrong a/f ratios, telling the ecu to add fuel when it's not necessary. = worse mileage

 

^ Which in turn starts to burn up your CATs