What made people put them on the sides?
Passenger space, putting the driver's seat off-center lets you add a rear view mirror (and prior to that being a thing, the ability to look over your shoulder- placing the driver in the middle gave them a rather bad blind spot right behind them) which has a better view of what's immediately behind the car. You also have to consider that, traditionally, the engine must be placed in the center of the car, given that it's the heaviest thing in the car. There isn't a lot of good ways to have the steering column fit around that without putting the engine on a mount behind the car, which can add cost and complexity.
Then again, some of the first cars where rather out there:
http://www.automotoportal.com/media/images/vijesti/070516008.jpg
http://www.speednik.com/files/2013/02/1905.jpg
http://www.americaslibrary.gov/assets/es/in/es_in_elwood_1_e.jpg
