As mule packer pointed out- the cutters and whetters are pulled really quickly after birth.
Hogs are MEAN animals. And yes, I mean farm pigs too. They are calmed by castration, just like bulls. If the males were not castrated, they would go berserk every time a sow went into season. And...... with a hog pen with say 6 sows in it???? That would mean a sow in heat about every week or 2 all year long, without fail.
Boars have a tendency to dig and root a lot. And, if they are breeders, they like to break loose to form harems. Or, fight, to form harems. If the hogs had their tusks and were penned up- you would have wounded and scarred up pigs unfit for market and quite a few extra dead hogs out of your pens.
With all this said, tusk size is always a toss in the dark as to whether the wild hog you shoot will have good tusks. In soft earth areas without lots of real hard roots, you can see some incredible tusks. In hard dirt, rock and areas with tough tree roots, hogs wear out their tusks at an amazing pace. Of the 13 pigs I have taken this year, 3 had tusks 3" or better. None had a tusk exposed longer than 3-1/4". My son John took a boar with 4.75" tusks in April. Amazing, awesome pig fangs.

In all reality, pretty rare for the areas I tend to hunt a lot.