Mostly it's just CYA for google since cycling is more dangerous than driving (due to the people driving), so there's more surface area for them to get sued.

But yeah

• turns and crossings that look safe on a map don't have very much data on whether they're actually safe, because google has a thousand times as much information about drivers than cyclists.
• google sometimes suggests routes that can't be traversed, legally or at all, by a bike. Same reason.
• sometimes google suggests avoiding something a bike doesn't actually have to worry about. This is actually the category of error I see the most: google sends you around something when you could simply walk your bike through it, or ride through it, because you're not a car.