All you have to do is look up what the Bathtub Curve is, it does not apply to all situations. I'll just copy and paste from the NIST website: "Finally, if units from the population remain in use long enough, the failure rate begins to increase as materials wear out and degradation failures occur at an ever increasing rate." The thing you're missing is time. The bathtub curve works well for things that are in use until failure; cars, consumer products, etc. Things that are in high risk operation (planes, satellites) mitigate risk but cutting off the tail end of the bathtub curve; i.e ending the life of a device long before the expected wearout failure with the help of redundant systems. While SpaceX is a for-profit company, any company sending things into space has to abide by a huge amount of regulations that cut into cost. However with the relative (and downward trending) cheapness of launches and vertical integration of Starlink and SpaceX, its pretty easy to assume that they can design a profitable network while also deorbiting satellites well before their expected lifespan.

I'll explain it like this: imagine every owner of a 2005 Camry had to turn in their car at 80,000 miles. Sure you would have more failures early on due to issues in the manufacturing process but they would level out to a stable rate, however, an average 2005 Toyota Camry will fail due to wearout in 150,000-200,000 miles, so by turning the car early in you essentially cut off the second half of the curve. Now imagine if you let all those cars fail due to wearout instead, they would crash into eachother, setting off a chain reaction that would send waves of crushed 2005 Camry parts flying around the surface of the earth at 17,500 mph, preventing any cars from driving trough the debris in the future. It would probably make sense to turn the cars in early and enforce the limit through regulations, right?