Hostile mobs can see and attack through blocks

I created a new survival world and started digging down into the dirt and making a staircase to find caves and ores when I fell down a cave and found a lot of hostile mobs. I quickly covered myself by placing cobblestone blocks around me and making my way back up to my staircase. I saw that the hostile mobs including zombies, zombie villagers, skeletons and creepers could attack me even though I was one block above them and completely surrounded by cobblestone blocks that I had placed. Since I was close to the cobblestone blocks, the zombies could hit me through the blocks and I was even blown up by a creeper even though I couldn’t see it.