Does your car go straight if you take your hands off the wheel on a bumpy road?
Mine wouldn't because it is a pile of junk. I would expect a reasonably well maintained car to go relatively straight on a motorway, otherwise why would you pay for tracking.
Why do you expect an aircraft to do so.
Because some Boffin who understands aerodynamics, designs in a nice amount of directional stability for me so I can fly and eat sandwiches at the same time.
FIS.