One of the main causes of infertility in men is low sperm motility. That is to say, the sperm are present and alive, but they have the swimming prowess of a toddler that’s afraid to lose its water wings.
In short, they just aren’t fast enough to reach the egg.
It’s hard to fault them, I can’t get to the front door fast enough for the FedEx guy not to leave my package in the bushes these days.