Homeowners who locked in a mortgage rate years ago have less incentive to sell and take on a new rate today, so they simply stay put longer than they might have in a different rate environment. That reduces the flow of resale homes onto the market even while demand from buyers, driven partly by population growth, stays steady.
New construction has not closed the gap either. Building takes years from land purchase to a finished, occupied home, so a jump in permits today does not show up as available inventory for a long while, which is why supply can stay tight even during a period when new listings are technically increasing.
Low inventory is a structural condition right now, not a temporary blip, and it is a big part of why a correctly priced home moves quickly. Get your hand-reviewed valuation from Joel Dyck.