I've hardly never hit any objects in the water, we have quite clean waters here, and no obstackles.
Hitting a sandbank happens now and then, when I ride in too low water say to get ashore when ending the session.
Doesnt matter, carbon can take this no problem, if you ride strapless in particular like leeuwen says.
But I have seen alu masts getting bent, wingfoilers quite often in fact.
Good thing is you can just throw it away and buy a new one, or have a spare, as they cost nothing
One of my carbon masts, used extremely often, are now more than 8 years old and still holding up just fine, I am happily amazed by their "lifetime" - havent had one getting "worn out" so not good anymore, and I have more then 300 sessions per year (not all on foil though, but most are on foil)
Even half as long lifetime would have been okay, so truly impressed.
I understand if one has rocks or logs or other stuff in their riding areas, or they are tough on their gear and can compromise on weight and performance - then low cost aluminium might be preferred indeed.
But you most often end up having the clumsy baseplate-board connection then - I have a baseplate on my wingfoil (eventhough carbon) and hate it, luckily it can stay fully assembled to board and 102 cm span foilwing in my car, otherwise I wouldnt have such thing.
Many brands offer adapters to their alumasts though, to better and faster connections, but not always possible.