The others pretty well covered it, but here are some other considerations:
Yes, you have a river on both sides of you - but this also means you have more exits as well. Say you're in top lane. And you get caught in the middle of the lane, between your lane opponent, and the Jungler coming up from river. Where do you go? You can't go forward, or you hit your laner. If you go to your own tower, the jungler can cut you off - you might try to hug the wall near the 2 top bushes but in the end they'll have a good chance of getting into range.
At mid lane, if they come from one side, you can always go out the other. Especially laners with a blink or dash that can use the river walls to escape.
And yes, the length of the lane makes the biggest difference - the "safe" zone takes up much more of the lane. Imagine this: Stand in the very center of Top or Bot lane. Then see how long it takes for you to walk back into your tower's aggro range. Do the same in mid lane. Mid lane is a much shorter walk.