Our body can perceive acceleration, but it is unable to perceive speed unless there is some external factor telling you about the speed. In a car or a train, the sound of the road or tracks or the speed of the wind tells us about the speed. In an aeroplane when it is cruising, the speed stays constant and hence the body is unable to perceive it. Moreover, the aeroplane is quite insulated, so the only sound that reaches us inside the plane consists of engine noise. The wind noise is unable to make it inside the plane. So, external forces don't give us the clue of the speed. Moreover, in other modes of transportation, we have objects (which can be used to gauge our speed) very close to us like trees, stones, houses, etc. But, in case of aeroplanes, we are so far away from any such stationary object that can help us gauge our speed.
So, in essence, no acceleration, good sound and wind insulation and absence of stationary object at reasonable distance make it difficult for us to gauge our speed in an aeroplane which is moving at more than 500 kph.