Engineers: Why isn't a roll pin lock used universally for linking to a motor shaft?

A solid pin has less give and has more resistance to shearing than a roll pin. Less vibration, less movement between the separate pieces means less wear.

Here is an excellent paper by Spirol that discusses some of the factors involved in making the mating component decision. It depends upon the materials, torque, expected length of service, breakaway torques, etc. My snow blower uses shear pins that are designed to break first if I hit something hard. They act like a fuse that protects the rest of the snowblower components. Good reading. (Really!)
How to Properly Pin a Shaft and Hub Assembly

My snowblower, Storm 3090 XP, has a dedicated spot on the control panel to hold 2 spare shear pins. (That is not a photo of my unit. I bought 6 and keep 2 in the holder and 4 in a labeled baggie on a shelf in the garage.)
I had to replace 2 when I ran over an oil pump my neighbor left outside his barn. It was buried under a foot of snow. I was going along, taking care of his driveway when BLAM! the machine shook and snow stopped shooting out of the chute.
I was able to drive it home, put in two replacement pins and was back at it in about 20 minutes. Had those pins not sheared who knows how much damage could have been done – broken auger, broken drive shaft, cracked transmission gears.

A roll pin is usually made of steel that is like spring steel, a steel with a high carbon content, and due to this, acts like a self retaining pin, but is too hard to use as a shear pin.
My grandparents had a coal stoker that had a gear reduction auger that fed the coal into the furnace. Since the stoker ran at such a low speed, the torque was very high and if a rock or something got into the coal, the drive shaft needed a soft steel pin that would shear without destroying the stoker. A roll pin would have been too strong for this.

Thanks for all of the excellent responses.

@LuckyGuy That link was really useful!

It seems my question wasn’t phrased very well. I’m less worried about the type of pin and why so many shafts don’t come pre-drilled to accommodate any type of pin. I should have been more clear on this point, but I was unaware of the wide variety of pin types (you guys have taught me a lot about it though which I am grateful for). It makes perfect sense that you want a failure point.

Is there a signficant downside to having a pin hole pre-drilled even if you plan to use a different type of connector? I guess on answer is that there are limitations on the hole diameter based on the shaft diameter (from reading @LuckyGuy‘s link), and that would necessarily limit pin holes to very tiny diameters on small motors which would be impractical.

Pre-drilling when you don’t intend to use it puts an unnecessary weak spot in the shaft. The cross section of the shaft is reduced at that point and can break.
Sometimes you do not know exactly where the motor will line up to the mating gear or pulley. In this case it is better to not have a pin and use some other kind of coupling like a set-screw or chuck type coupling so the mating gear or pulley can be lined up first and then tightened in place.
There are so many options. vibration reduction, strength, placement, service access, assembly access, etc. If you take apart a few mechanical things and look closely at what’s there you can appreciate how the designers had to make decisions about every component.
And now with 3D printing of metals possible you can create parts that could never be made using conventional machining methods. That adds another dimension to the design process.

@LuckyGuy Thanks for the great answer!