Let us remember that Robinson Crusoe has been left shipwrecked on an island which he knows nothing about. Although he has scouted the territory and does not see any sign of other humans or of dangerous wild beasts, he is very unsure of his position, and, understandably, feels rather vulnerable as he is alone, isolated and stranded. Therefore perhaps we can understand why he feels the need to do everything he can to protect himself. Note what he says when he begins to think of the kind of shelter he needs to build himself:
My thoughts were now wholly employed about securing myself against either savages, if any should appear, or wild beasts, if any were in the island; and I had many thoughts of the method how to do this, and what kind of dwelling to make - whether I should make me a cave in the earth, or a tent upon the earth; and, in short, I resolved upon both; the manner and description of which, it may not be improper to give an account of.
He is trying to protect himself from unseen dangers as well as seen dangers, and therefore, because of this, perhaps we can understand the lengths that he goes to in order to accomplish this. After all, if we were in a similar situation when we face so much that is unknown, we would want to take every measure to defend and protect ourselves from threats that are perhaps unseen but nevertheless potential worries to us. The peace of mind that such defensive fortifications would give us would be immeasurable in such a vulnerable position.