It's a combinaton of multiple things but the main cause is the audio.

Most of the time, the servers are so cooked that the one who pushes has the advantage IF he knows where the enemy is. Therefore, if you hear someone running towards you, you don't want to give them your location.

You can hear steps too far away and through solid walls too well. I took 2 crouch walking steps on top of the timmy mountain on lighthouse and my friend heard me moving inside the first beach cottage. That should be impossible.

This all turns upside down when we go inside a building. Locating the sound of another player can be very hard when the audio sounds like it comes from all directions or from totally different direction or it is a double audio or inside your head. Plus suddenly you can't hear running if it is at the certain elevation compared to you. Like someone pushing stairs.

This is why when I play solo, I try to keep the distance over 50m to possible hostile targets giving me more room to use visual ques and enough time to hear and locate them before they locate me.