Because the זהר says so

 

.    Parshas Hashavua ויקהל The זהר פ' ויקהל דף קצ"ה writes, " All nights have exactly  12 hours. Even in the winter months there are only 12 hours of night. Tthe extra hours of night are considered daytime. The מעשה ארג*,based on this זהר, claims that when the משנה פ"א מ"ב מס' ברכות  refers to שעות it means שעות שוות (equal timing -60 minutes to the hour nightime & daytime winter & summer) His claim is against, the Rambam & all the Poskim  who all hold that the Mishna is referring to שעות זמניות (daylight of the day is divided into 12 this equals a Halachic hour for  daytime Halachos and same is done for night time.) When the משנה says The בני מלכים get up at ג' שעות It depends what time of the year it is (שעות זמניות). The מעשה ארג is so convinced to be right that he writes, Anyone disagreeing with  him is like disagreeing with the שכינה, being the זהר says so. The אגרות משה או"ח ח"ב :כ strongly opposes such a statement and claims on the contrary, Those having doubts on all the  Poskim are the ones who are having ח"ו doubts on the שכינה. רבי יצחק אייזיק יהודה יחיאל ספרין (כ"ה בשבט תקס"ו, 1806 - י' באייר תרל"ד, 1874) היה האדמו"ר השני מקומרנא.*