A screen on the back of a camera is lit up by the environment, so it needs to be extremely bright to work properly in direct sunlight while also having zero bac

A screen on the back of a camera is lit up by the environment, so it needs to be extremely bright to work properly in direct sunlight while also having zero backlight bleed in near darkness. Viewfinders solve this issue. The problem with live preview from a digital sensor is that the sensor needs to be able to stream the entire image constantly. Early sensors were not designed for this, and would often overheat quite quickly. It also needs to update often enough to not be jittery, and in low light conditions early digital sensors just weren't sensitive enough. You also need an extremely high-resolution viewfinder screen to provide enough detail for things like focus. Those didn't exist yet either. One thing to keep in mind is that it was definitely possible: consumer cameras did it quite early on! But the quality was quite poor: good enough for holiday snapshots, but not nearly good enough for professional photographers. Sticking to the DSLR approach was a no-brainer for them. Also, a (D)SLR doesn't split light: a mirror flips down in front of the sensor to reflect the light into the viewfinder. When you take an image the mirror flips up, allowing light to fall onto the sensor.