A Formula One car is a delicate balance of synergistic forces. Get it right, and you have winner. Get it wrong, and it's back to the drawing board (remember the walrus nosed Williams?). It all starts at the initial design of the car, when all the components are carefully designed, each in a specific relationship to each other. That basic design is frozen at some point, and then major construction begins. A schedule has to be met. And the heart of the car is the chassis, everything relates to it. So the carbon fiber chassis is designed, and usually, no major changes to it occur during the season. Hopefully, and most of the time, just small adjustments are made to refine the new car. Ballast may be shifted, wing may be added or reduced to the front or rear, careful tweaking of the suspension.
And a carbon fiber chassis is a lot of work to manufacture. It's a complex three dimensional shape with very close tolerances. I don't know how many rejects happen when attempting to manufacture a carbon fiber chassis, but not all come out of the autoclave passing final inspection. As well, a newly designed chassis has to go through the FIA's rigorous crash testing. As far as I know, each team manufacture just a small number of race chassis, maybe only five or six per driver. Maybe even less.
Carbon fiber, unlike aluminum or most metals, doesn't fatigue and fail. So unless it's subjected to harsh envoronments or an impact, or rough handling, there's no reason why a chassis can't be used until it becomes obsolete.
Of course, all the other parts that are attached to the chassis may be easily changed at a moment's notice, even the driver.
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