Because that would be expensive to have to recall all bills and coins and remint them ever 4-8 years.
You wouldn't actually have to recall them. They would stay in circulation for as long as their normal lifespan.
New money gets minted each year to replace that taken out of circulation. You'd just have to change the design every 4-8 years.
If this was a thing, and say it started when Bush was elected, there could still be notes in circulation with bill on them even though Obama is the president.
Regularly re-designing money is also a very very good way of limiting counterfeiting, as counterfeiters would have to regularly keep up to date with designs.