Not that what Solar Mike is saying is wrong, but to add to his post ...
There's three problems with what you're suggesting:
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 ...
Because it can, or does, make fitting belts more difficult.
A walk blocker won't affect tension.
And belts only tend to come off the pulleys for two reasons (imhe) a bearing has failed and the pulley sits at an angle or the belt is delaminating.