There are two reasons for that: Firstly, the tarmac (asphalt) softens, making it harder for aircraft to build up to airspeed, and secondly hotter air is thinner, meaning that aircraft can't get enough lift in taking off and landing. It's not a problem when cruising. https://www.cntraveler.com/stories/2016-06-20/its-so-hot-some-planes-cant-fly-heres-why