You are right, but the enrichment of those asteroids in platinum-group metals is very small in comparison with the corresponding depletion of the surface of the planets.
While the depletion has reduced the amount of platinum-group metals by many thousands of times, the enrichment only increases their concentration a few times over the average concentration.
The reason is that the planet cores still include iron, one of the main 10 elements of the universe, the least abundant of which is many times more abundant than all the other elements combined.
The enrichment consists only in the removal of the magnesium, silicon and oxygen from the planet core, while the iron stays there.
Even with all the platinum-group elements in the core, their abundance cannot increase beyond the limit imposed by the ratios between their average abundances and that of iron (actually their abundances become a little larger than those ratios, because a bigger fraction of iron remains oxidized in the mantle than the corresponding fraction of platinum-group metals, but the difference above the average ratios remains very small).
While the depletion has reduced the amount of platinum-group metals by many thousands of times, the enrichment only increases their concentration a few times over the average concentration.
The reason is that the planet cores still include iron, one of the main 10 elements of the universe, the least abundant of which is many times more abundant than all the other elements combined.
The enrichment consists only in the removal of the magnesium, silicon and oxygen from the planet core, while the iron stays there.
Even with all the platinum-group elements in the core, their abundance cannot increase beyond the limit imposed by the ratios between their average abundances and that of iron (actually their abundances become a little larger than those ratios, because a bigger fraction of iron remains oxidized in the mantle than the corresponding fraction of platinum-group metals, but the difference above the average ratios remains very small).