IAS is the pilot's direct indication of EAS - which is essentially a direct measure of dynamic pressure. The wings react to dynamic pressure. At takeoff and landing speeds there is almost no difference between IAS and EAS. Any difference will be accounted for and the speeds published in IAS for a pilot's reference will reflect those differences.
From an engineer's POV, the airspeed indicator should be calibrated in units of pressure such as millibars. But from a pilot's POV, pressure doesn't make sense. They've calibrated the pressure indications for a standard day at SL, meaning that under those conditions, IAS will equal TAS. As altitude increases, IAS will decrease compared to TAS.