Why underage children easily access porn sites on the internet?

Alright, this question is a very big and this is one question that all parents should be asking themselves, "why under age children access porn sites even under the watch of their own parents. Ok, I will not want to start blaming the parents for not watching these children well, I suppose the truth of the matter is that internet is everywhere right now, inside your car, your house, in schools. We are having internet networks everywhere and everything we are having in our houses like the phone you know everything. I think there should be a measure some of these major social media sites should put in place to restrict some kind of content been posted on their site. Imagine an adult posting something that is very erotic in his own facebook page not even minding that probably he/she may have teenagers or underage children in his page. This people will easily see this things seriously. There should be a tag that should restrict these children from seeing these, because if they keep carrying their parents phone and the next they open it's just go straight to the porn site. These things is affecting their mind and body and it has a very great negative effects on their lifes in the nearest future so I think as much as possible parent should tell their children the negative effect of them engaging in p********** generally then they should put measures to make sure that these children regulate the kind of things and even when they have the opportunity they will not want to click these things and because they know the effect these things we'll be having on them in the near future.