Modern waste vents are usually bare plastic pipes extending a short distance up through the roof, with no cap. Compare the vents in this image:
Judging by the apparent age of the neighborhood, the waste vent on the left is probably nothing more than a stub of ABS pipe. PVC (white) is also sometimes used. Older homes can have metal (copper, cast iron) or even clay vent pipes, but they're no more complex, just a different material. These pipes drop straight down into your drain system, so there's no harm done by rainwater or small debris falling into the vent.
On the other hand, vents for appliances are designed specifically to protect the system from any ingress of water, dirt, debris, small animals, and so on. They're also usually attached to larger-diameter ducts, as opposed to smaller-diameter vent pipes, because they need to allow for more airflow. Sewer ventilation is very low-flow, it's just preventing the gradual buildup of gases produced through natural processes, so it doesn't need a large pipe.
The vent in the middle is larger than the waste vent because a furnace or boiler puts out combustion byproducts at a much higher rate than decomposition produces sewer gas. You can see that the attic fan on the right has the largest duct by far, and that makes sense because it's attached to a system that's designed specifically to move large volumes of air through the home.
There are also reasons to be particular about how you vent appliances, since this can impact how well they operate, home energy efficiency, etc. There's no reason to be particular about how your sewer pipes are vented, as long as the sewer gas can't flow back into the home, which is achieved easily with traps in the system, far from the vents. All that's needed at the vent end is a simple, inexpensive, unobtrusive pipe stub and a good seal with the roof.