Why don't turbofan engines have shrouds affixed to the fan blades?
This has actually been done, at least on the CFM-56 series at some point.

The image is taken from here, the article is about the F108 engine which is the US military version of the CFM-56.
In newer versions of the engine the blade-tip shrouds have been replaced by mid-span shrouds as below (from Wikipedia), or have been removed.

As pointed out by Camille Goudeseune, turbofans have to accomodate a lot of situations, and a blade-tip shroud adds a lot of mass and therefore increases loads in the blade, further reducing fatigue life.
Among the advantages of this configuration that were not mentioned in the question or in the other answer is that shrouds help reduce vibration amplitudes through frictional damping. This explains why mid-span shrouds are still in play in several engines, while these do not participate in reducing blade-tip losses.
Finally, while this configuration is seemingly not used anymore on fan blades, it is still used in other parts of modern engines, specifically the turbine module.
As an example, the picture below is from the GE9X website, see the Low Pressure Turbine blades.