I'm just using logic to answer this question.
We see many times in the books wands can actually be targeted by movement spells, just like every other object with a physical manifestation.
This said, i think the logic behind the "Protego" works fine.
For example, during the duel between Dumbledore and Voldemort in the Ministry of Magic not a single word is spelled while the two of them keep shooting every sort of wizardry to each other, and countering the opponent moves as well.
I think it's correct to presume they are constantly countering that kind of attack too; considering the fact that losing the wand means sure death or at least demise in a duel, preventing this should be one of the first safety points to take care of.
If i can make an off-topic example to show a similar logic in another fictional universe, it is the following. What would prevent a Force user (SW universe) to shut the opponent's lightsaber off with the Force?
Clearly nothing, it's a physical object and thus subjected to Force manipulation. The canon answer is: both duelers are constantly keeping their weapon controlled and on with the Force.
In the same way, wizards must , or at least should, be constantly protecting the vessel of their power.
EDIT: i also came up with another theory. In the books we see certain kind of objects immune to the Accio spell (E.G. Horcruxes, Harry's Invisibility Cloak..)
We know they are powerful magic items, but they must have been crafted by someone.
This means, whoever imbued them with their respective abilities, has also given particular qualities, such as regeneration for Horcruxes and durability for the Invisibility spell on the coak.
Thus, what should stop a good mage to enchant his own wand not to be subjected to such a powerful threat? Think it once, make your magic, end. Easy done, given you have enough power to cast such a spell.