Several reasons for this:
1) Light doesn't really go far underwater, so you could maybe see a few dozen metres. See this picture, for example:
View attachment 3719
Doesn't exactly goes that far, now, does it? This wouldn't be an issue except for the second point:
2) To quote Seaman Jones in Red October, 'The catch is, a boat this big doesn't exactly stop on a dime... and if we're too close, we'll drift right into the back of him.'
A nuclear submarine is a 5000 to 30000 tonne beast that, at regular speeds, goes 5 to 10 metres per second. It is optimized to go swiftly into the water and has very little in terms of control surfaces, no such thing as brakes and is usually having its propeller linked mechanically to turbines. So it takes TIME to slow down, quite some time. Therefore, you now have to consider the OODA loop: Observe, Orient, Decide, Act.
Observe: the conditions are such that purely passive detection of the underwater environment are extremely limited in range.
Orient: Visual observation isn't exactly a perfect science, and you have to identify in a blurry picture what could be a mountain.
Decide: That can be fast, emergency reaction time onboard a submarine tends to be drilled to five seconds or less for critical actions.
Act: Emergency back power isn't instant in any case.
Therefore, bump.