Why do car companies see the need to design completely different models of cars every year?

To get some one to trade in an older one.

Lets face it, if the 1963 Ford Fairlane was still being made a lot of car owners would still be driving one that was twenty years old.

Car manufacturers design and bring out a new model every year for the same reason that cell phone manufacturers bring out a "new and improved" version of their phone every year - people seem to want a "new" model of their car or phone, and they're willing to pay for it.

That was a "Marketing Con". A new year should be a new car design. People want to be seen driving the latest and greatest in cars. The only way that was possible is by changing the body.  

Why not do it like aviation companies?  Why not indeed.

Ask yourself this:  How many people get financing to buy a brand new 767 every few years?  How many times have Boeing or Airbus announced that their remaining 2019 models are marked down to last year's prices?  

That's why.  

Customer vanity demands it.

So people will have a NEED to buy a new car.  NO ONE will buy a "new" Ford Explorer now if the body has not been redesigned in 20 years.

False assertion. They don't manufacture completely different models every year.

Car models are not redesigned every year.  Sure, there may be some minor trim changes, availability of different options, or colours, etc. between model years but they are usually very minor changes.

Depending on the manufacturer, models usually have a 5-8 year life cycle with usually a refresh somewhere in the middle to introduce new technologies or alter the styling to suit changing consumer trends.

Why do professional sports teams have multiple jersey's in a year?

- home, away, halloween, saint patty's day, and all the other special events....

Because the more variety of jerseys they have, the more likely the die hard fans will buy a new jersey....

Cars are no different.... people are more likely to buy a new car when there is a model change.  That's why they completely change the look every 4-5 years. 

They don’t.  Typically they make some minor changes year-to-year based on efficiency or cost. Maybe a bigger side mirror or a slight change in trim.  If some part or system is experiencing a high failure rate they may redesign/replace just that piece.  Redesigns are typically every 4-6 years where it can sometimes produce a radically different design.  And I’ve never seen a car maker redesign every single car in the same year. It’s usually 1/4 to 1/6 of their models are redesigned in a given year - something that their engineering and design departments are equipped to handle. 

Airplanes are commercial vehicles with lifespans of 40-60 years so change is slower. Cars are consumer vehicles subject to consumer tastes which fluctuates more often.