When someone enquires through a property portal, they are rarely enquiring about one property. They send three or four enquiries in the same sitting, and the agent who responds first gets the viewing. Everyone else is competing for second place on a property the applicant has already seen.
Most agents know this and still lose on it, because enquiries arrive while negotiators are out on viewings. The enquiry sits in an inbox until someone is back at a desk, by which point the applicant has booked elsewhere.
The second problem is quieter and costs more. Vendors and landlords who hear nothing between milestones assume nothing is happening, and that is where instructions get withdrawn and fees get renegotiated.