The only explanation for the cycle routes Google spits out- the only one- is that it's taking money from the motoring industry to make cycling less appealing. OK, that might not be the only explanation but it is by far the most fun, entirely baseless conspiracy theory available so let's run with that. It's possible the routes they select would be the fastest if the bikes were cycling themselves and traffic didn't exist. But as it stands, following Google instructions will often lead to a slow and stressful journey. Cycling websites tend to have alternate, useful cross-town journeys, otherwise "parallel to the high street" often does the trick.