Because you don’t see God with your mind’s eyes as He was real.
We hear some people say: prayer us makes quickly tired. Why? Because you don’t see God with your mind’s eyes as He was real. Watch Him always with the eyes of the mind and heart and then you will be able to pray all night without being tired. A night to say the least! You will be able to pray three nights and three days in a row and you will not be tired. Remember about the stylites. They prayed for many years on a pillar, conquering their body that tended to be idle. And you find a few hours of prayer, or even one, difficult!
(Translation from: Sfântul Ioan de Kronstadt, Viaţa mea în Hristos, Editura Sophia, Bucureşti, 2005, pp. 111 - 112)