That is because the are safety factors and conservative assumptions built into that lifespan to ensure that even if you moderately abuse the bottles (e.g. washing in hot water and detergent, exposing to sunlight or high heat, et cetera) they will still have integrity. Fortunately, a polypropylene bottle containing 30-40 psi of water and dissociated CO[sub]2[/sub] is unlikely to present a substantial harm even if it does rupture and with the thickness of the SodaStream bottles, I would expect that they could probably retain >200 psi in the new condition. (A much thinner wall standard 2 litre bottle can retain about 100 psi pressure in new condition.)
The answer is that you can probably get away with using these bottles indefinitely as long as you do not abuse them or subject them to accelerated wear conditions, and if they do fail, they’re unlikely to pose the kind of hazard that high pressure (3000 psi) air or volatile gas would. SodaStream is essentially covering themselves against a frivilous damage suit in case a bottle does rupture and spook some clueless moron. But with real pressure vessels, they are periodically proof loaded and inspected to ensure integrity, and correct process is to scrap them if they show significant wear, damage, or defect.