I didn't say there always is sperm in pre-cum, I said there can always be sperm in pre-cum.
The potential is there and you have no way of knowing until it gets you pregnant, why bother taking the risk.
I don't know what sites you have looked at that say pre-cum doesn't contain sperm, but there is always a good chance that it does.
Pre-cum is a mans natural lubricant that travels via the same tubes from where the sperm is made, there is nothing to stop it collecting a few million sperm on the way.