Try to imagine a bunch of poachers holding an elephant still while sawing off the tusks.. won't happen. It's not like they would grow new tusks next year anyway. As for killing off the part of the population carrying the "tusk" gene, they don't think in those terms.
Poaching is illegal. All practical issues with sedation and humane tusk removal aside: these guys need to work fast. They shoot the animal, race over to it, hack the tusks off at breakneck speed, then disappear.
Penalties for poaching are generally paid with lead.
I'm surprised that poachers use firearms rather than poisons or sedatives, risking announcing their position. A very powerful (and loud) cartridge is necessary to kill an elephant.
Also, I thought most African elephants in managed wildlife areas have their tusks cut by wildlife officials to deter poaching and save animal lives. If this were the case, perhaps attraction in mating also no longer includes tusks as a desirable characteristic since few/no individuals possess it any longer.
It also takes massive quantities of powerful poisons and sedatives to drop an elephant. And either of those options can take time. And cost.
As to potentially giving your position away due to gunfire, the savannah is a big place and relatively flat. You can be hours away from the next human being. You can see them coming from miles away. And it makes it difficult to tell where sounds are coming from. So while you may hear the shot, you're not going to be able to tell exactly which direction it came from.
Also, you only need to be far enough away to not be caught.
That's back when firearms were overbuilt and used low chamber pressures because they didn't have finite element analysis simulation software and their materials weren't consistent enough to not blow up, or have to weigh 200 lbs. Dumping great energy into a mass is what matters.
You can heave a cannonball at an elephant with a potato gun, but it won't do much. .45-70 Government isn't a Big 5 cartridge despite being almost 12 mm across because of its antique origins and low chamber pressures.
A tiny FMJ high velocity round like a 5.7 wouldn't do much despite having the penetrating power of being essentially a supersonic "needle".
.338 Lapua requires a 5400+ J load to legally hunt B5 in parts of Africa. Otherwise, a 12.7x108 or .50 BMG would do the trick. Anti-matériel rifles are a lot easier to acquire than useless, antique elephant guns. Barrett.net