Because film captures details in molecules while a CCD sensor in your digital camera uses picture elements much, much, much larger than molecules.
Or, because it's analog. It's very similar to sound recordings. Whether magnetic or not (say, in vinyl) the resolution of the captured signal can approach the molecular. Digital has a discreet resolution limited by the capturing equipment; for example, CDs reproduce the volume in a sound recording only 44,100 times per second. But a magnetic tape has a resolution limited only by the number of molecules passing the recording head every second (the more [i.e. the faster the tape], the better.)
Or, because it's analog. It's very similar to sound recordings. Whether magnetic or not (say, in vinyl) the resolution of the captured signal can approach the molecular. Digital has a discreet resolution limited by the capturing equipment; for example, CDs reproduce the volume in a sound recording only 44,100 times per second. But a magnetic tape has a resolution limited only by the number of molecules passing the recording head every second (the more [i.e. the faster the tape], the better.)