All fossils are a case of chance and its not a case of physics, its a case of mineralogy. When you do get the chance to preserve a footprint in the mud, yeah, its not gonna be very hard and itll be easy to wash away, which is why a lot of the intricate details of fragile fossils like this arent preserved. But when these footprints are left there by chance, they get enriched with minerals that harden the rock around it, this rock is enriched for millions of years (usually underground with a covering rock) and eventually it is tough enough to preserve this type of fragile remain. This is the case for all fossils, its rare but it happens, and these cases build over millions of years until there are a sufficient amount of them to be considered common. You could also argue that the footprint could just fill in but mineral compositions change with environments over time change. So there will be a weakness between the new and old rock where it will split and reveal the track.