There is a confusion here - if a share price is said to follow a random walk it doesn't mean its current value is actually random (in the sense that it is not based on anything)- it just means that its value in the future is unpredictable.

There is a reason why a share price is at a certain level at any given point in time - predictions of future profits, macro economic conditions etc. no one doubts that - the current share price is not random. But the point is that the future share price is not predictable because all available information about future conditions, future profits etc. has already been included in the share price now. The only thing that can therefore change the market's valuation is NEW information. And when that new information comes, then the share price will change almost instantaneously (depending on how efficient the market is). So the theory is that you can't predict changes in advance because you don't know what the new information will be - you have to wait until it actually arrives, and by then its too late to make a prediction - the market changes straight away...