Several reasons .... a 'quality' tank was installed, one that welded up by an 'artist' who did 'full penetration' weld (very difficult in thin metal) without overheating the base metal, didnt have many weld laps, slag, etc. in the weld. The welder may have also passivated the tank when the welding was completed ... most ss tanks for boats arent passivated. The pin holes are usually IN the weld area, especially where the weld is not 'smooth' and the corrosion propagates into the 'irregularities' from substandard welding.
Other may be that you are in an area that has a newer municipal water system that has its source of 'clean' source water with relatively low 'bio-burden' that requires very little chlorine concentration (.25-.5ppm) to keep it 'sanitary' ... instead of some areas that require heavy dosage (1-3+ppm), etc. ... or you may be using 'well water'.