A particular gate can't accept every type of aircraft. The gates you see open may not be able to accept your airplane type and/or they are one of the few gates that can accept other types, i.e. widebodies, which will be arriving soon. There's also equipment issues with gates where the gate's GPU (ground power) or PC Air (pre-conditioned air) could be down so you can't park an airplane with inoperative APU there, etc., which adds complexity to the situation.
It's a big jigsaw puzzle and, at least at my airline, there is a person at each hub whose only job it is to control the gate assignments. I sat behind the gate controller for a few minutes at one of our hubs a few months ago. She was non-stop coordinating gates, gate swaps, and other gate-related issues.
The real problem is that many hub airports are behind demand in their gate construction. Many of them have projects either ongoing or in the planning stages to add additional gates.