Because this is what real trucks are supposed to have.
In case you haven't noticed, the engine downsizing trend for cars and crossovers has also struck pickup trucks, such as the Ford F-150. Sure, you can still get a new F-150 with a V8, but the twin-turbo V6 is more powerful and more fuel efficient. Despite this, more traditional truck owners still prefer the V8. According to The Fast Lane Truck, GM has no intention of ditching the V8 anytime soon. In fact, an anonymous source told the website that the redesigned 2019 GMC Sierra 1500 Denali will retain the 6.2-liter V8.
Since the Silverado 1500 is the Sierra's corporate twin, it's fair to assume the V8 will be retained as well. The source claims they were attending a recent dealership meeting held by GM and that no photos or videos were allowed. It was there the 2019 Sierra Denali was on display. Although dealer representatives weren't allowed to see the supposed "revolutionary" bed features, the truck's hood was open, exposing the V8. Obviously this won't be the only engine offered and it's entirely possible a smaller turbocharged engine will power lower trims. We don't have an exact date as to when GM plans to reveal both trucks, but this January at Detroit is definitely one possibility.
We've also heard that Ram will reveal a redesigned full-size truck of its own in January, and it too will likely continue with a V8 option. GM also plans to stick with a mostly steel construction as opposed to the Ford F-150's lightweight aluminum focus. Sources are also claiming the new GM full-size trucks will have an evolutionary exterior design that'll appeal to longtime owners, but the interior is said to be undergoing more dramatic upgrades, such as a larger touchscreen that bears a strong resemblance to an iPad. GM is currently testing the trucks on public roads, indicating development is moving along nicely.