Why do digital cameras have shutter speeds?

You need the sensor to be exposed to a bombardment of photons to be stimulated and to gather the image data.

It needs to be kept open for some time frame.

It's a bit reductive, but if you only kept the shutter open for let's say, the Planck time you would be lucky to catch one photon on the sensor. So the time has to be greater than that, but obviously less than infinity.

Because:

• some still have shutters, especially pro level cameras
• nearly all information about photography involves shutter speed, including software and book tables for calculating exposure, in addition to how cameras communicate with external flash.
• pertaining to the previous point, there are still flashes that are set by shutter speed.
• there are still a lot of film cameras use out there

But really, the simple answer is that shutter speed is just what we call exposure time, for historical reasons. You could call it something else, but it'd mean the same thing. Why rename it, when it's still literally shutter speed in much of prosumer and high-end photography?