Heat transfer at the surface
When cold meat contacts a hot pan, heat flows into the surface by conduction. The temperature rise at the meat surface is rapid at first (seconds) because there is a large temperature difference driving the transfer (pan at 450F, meat at 40F). As the surface temperature climbs, the rate of heat transfer slows because the delta narrows. Meanwhile, heat diffuses inward into the meat, so the 'hot layer' at the surface grows thicker over time, further reducing the surface temperature gradient.
The Maillard window (typically 300-350F for beef) is the zone where flavor compounds form rapidly. If you sear at lower temperatures, you miss this window; if the meat is already warm (reverse sear), you hit the window almost immediately and can sear very briefly.
Why sear duration matters
The surface temperature asymptotes toward the pan temperature, never quite reaching it in normal cooking. A 2-3 minute sear on a 500F pan raises the surface to roughly 130-150F, enough for a good Maillard crust. Longer times (5+ minutes) don't improve browning much but do allow more heat to diffuse inward, risking overcooked edges on thinner cuts. This is why the reverse sear, which starts at target temperature, needs only 90-120 seconds.