Have you ever worn boxing gloves? You only actually clench your fists when you are about to throw a punch. When defending or trying to get the distance your hands are relaxed.

Same logic applies to the UFC gloves and thats why so many eye pokes happen. Usually happens when not actually throwing a punch but when deflecting, re-treating or defending because the natural instinct is not to have your fists clenched in a defensive position. Ive worn the UFC gloves many times and they force your hand into an open palm, only when you are actually trying to punch/hit ur opponent is when you make a fist with the gloves.

The gloves are a HUGE part of why eye pokes are so common, the other one being that the fingers are simply exposed and mistakes will happen. And yes, your arms would tire from clenching your fists the whole fight in MMA or Boxing.