Replacement isn't perfect. See scars.
As you age, your body starts to have hormone imbalances that disrupt optimal immune and cardiovascular function, so you tend to succumb to disease and heart problems more easily.
Also, a property of Eukaryote cells is that their DNA isn't circular, so cell division always chops off the last remaining bit because the DNA polymerase isn't able to reach the last few. Those are then discarded, and the DNA gets shorter. When you run out of the Telomeres at the end, your cell refuses to divide. For humans, it's around 50 divisions.
There are ways around both, but it's not trivial. Humans initially were evolved for short life spans, and relatively quick reproduction rates, focusing on quantity of life rather than quality, as it's more cost effective genetically that way, rather than investing a ton of energy into prolonging life which might just be cut short due to some silly accident, or a predator coming and eating you. Other animals with fairly safe lives tend to do the opposite, focusing on prolonging life so they have more time to breed. (turtles and elephants for example)
And genetic damage due to exposure to the elements. Slowly you deviate more and more from your "original" genetics, because of cosmic rays, carcinogens, and general ionizing radiation shooting holes in your DNA, disrupting functions. Random mutation simply copying wrongly also builds up the mistakes. Eventually, the cells just simply don't function properly anymore; maybe a collagen making cell makes defective collagen now, so it and its descendants will make defective collagen.
There's lots of factors really. But fundamentally, the body isn't meant to last forever.