The Pak Deep State has no problem with a porous border with Iran for a reason. The West can't say much for they are behind the unrest in Baluchistan. It's an "occupational hazard" for them....
While I agree with you, it's more complicated, especially on the Iranian side, they have built a large wall yet still there's comings and goings, the weight of the question lays fully on the Iranian side.
On the Pakistani side, it was lack of important to the Iranian border because we don't consider it as a threat, brotherly this and that, baring terror activities but we had other fish to fry.
There was no border fence on the Pakistan side, and it was too long and difficult to control without mutual mechanisms, which just did not exist, we're still building on a suitable formula for mutual border security.
Plus, this region of Pakistan is least developed so economic flows with Iran allows for a local economy to exist, be it smuggling etc so more a case of looking the other way and let the people make a living. of course others will also skim from the top.
But Pakistan did not shout about the issues much, thinking it'l settle down eventually. It's the Iranian who've been crying, building walls yet still lacked control, was that willful? I would say it was.
there’s no shortage stop fooling people. 
