Is it the sensor to blame? Because film-based pictures do not need sharpening.
There's two mistaken statements in your simple question: first, that sharpening is always needed for digital photographs, and second, that it's not needed in film.
Let's start with the second. Film actually isn't fundamentally different here. Scanned photos often benefit from digital sharpening to match the output medium. But not even digital: the common "unsharp mask" digital sharpening technique is called that because it is based on an analog technique used with film.
It is true, though, that there are some things about digital sensors that mean that sharpening is a normal part of the workflow. One of these is that most digital cameras include an antialiasing filter to help reduce moire. That works by introducing blur, so sharpening counteracts that. Also, sharpening needs to be part of interpolation when using a Bayer-pattern sensor. Digital leaves you the flexibility of leaving that sharpening for the end of your workflow rather than locking it in. Depending on your sensibilities, though, you might decide that sharpening isn't needed at all.
But also, digital photography has ushered in trends that aren't related to needing sharpness in an absolute sense but are definitely related to an obsession with it. People tend to view images on computer screens, in high resolution, and when critiquing them, zoom in to 1:1 pixel view. This is like scanning over every photo with an extreme magnifying glass; in the film days, some people did this, but most people recognized that you could appreciate a photo without doing it. Now, sometimes, people get an obsession with a certain sort of technical perfection in their photographs (often to the detriment of artistic qualities, or even more complicated technical qualities).