Why don't car manufacturers make camshaft pulleys with walk blockers as a safety measure?

Not that what Solar Mike is saying is wrong, but to add to his post ...

There's three problems with what you're suggesting:

1. Some companies DO put the walk blocker on the camshaft pulley. Look at the Honda J35 V6 (goes for the other J3x series v6 engines as well) for instance:

The rear cam has the walk blocker on it. This isn't a one off either. More than just Honda has done this.

2. You only need them on one pulley to be effective. It doesn't matter which position they put the walk blocker at, it just needs to be on there. The width of the belt prevents much of this from happening as long as everything is aligned correctly (cam/crank gears, tensioner, idler, water pump, etc).

3. The only real reason many have one in the first place is because they use plastic timing belt covers (like the J35). If you didn't have one there, the timing belt would eat through the cover if it tried to walk off one of the pulleys. If the manufacturer designed the cover so it rode close enough to any given pulley and it was made out of metal, you wouldn't even need one on any of the gears. Plastic is by far easier/cheaper to make then metal, so I completely understand why most wouldn't go this route.

NOTE: I've never considered it called a "walk blocker" before, but I guess the term works so no issues from me.