Its because they do it for all the wrong reasons.
Anyone who switches well out of range is just trying to fight the other way around to see if there are any gains for or against, or to put their opponent off their game or rhythm after losing ground themselves.You'd have to be a great ambidextrous fighter with deep understanding to do that at will and gain each time.
If you switch in range without them first reacting you are going to leave one hell of a gap up the middle at one point in time mid switch and you are going to be square to them and can toppled backwards easily if they are sharp or if you try it twice. If they do a rare flying or leaping hook you are going to get caught square and probably down for a count you want to be able to cover that gap by range or control of them.
If you switch feet and duck out of a situation where they are already attacking and moving in you can go from inside their arms to outside their lead arm while attacking over it with your new lead arm then under it with your rear as you close them up.that way is safe cause you are switching out away from their rear arm and dealing with their lead arm continually.
Switching out from being trapped on the ropes is an old art too.Or pull them in toward you(so they react by pulling away) then you go with them,or the reverse where you push on them bodily they lean into you in reaction then you switch places with them using their momentum or cover up and slide your lead leg backwards crouch sit on the middle rope to give yourself room and switch feet and duck out under his lead arm and rip one up the inside as you are leaving.
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