This is exactly the question the T'rumas Hadeshen was addressing.
ברוב הקהילות נוהגין בימים ארוכים בימי הקיץ לקרות קריאת שמע של ערבית, ולהתפלל תפילת ערבית, ג' או ד' שעות לפני צאת הכוכבים. אם יש שום ישוב או טעם למנהג זה, כי גם הרבה תלמידי חכמים עם ההמון עם במנהג זה?
To paraphrase, "is there any justification for this widespread practice?" And his answer in short is "no". It opens with the words "...דאין בידינו כלל למצוא ישוב וטעם" - "we are totally unable to find a justification or reason..."
He does bring up the possibility(!) that the calculation of p'lag hamincha, which is subtracted from nightfall, could come out significantly earlier for some people, if they are calculating "hours" as twelfths of daylight rather than 60 minute periods, but dismisses this as helpful since no such long hours would be long enough to get p'lag hamincha 180 minutes before nightfall.
Short of justifying the extremely early arvis and k'ri'as sh'ma, he does hypothesize as to how it gained such adoption by respectable people. The leadership in places where night fell very late in the Summer (e.g. קרימ"ש) saw that people's inability to eat before shabas due to a safeguard emplaced in the Ga'on era was compelling them to forgo t'fila. So they saw fit to allow the early t'fila to go on.
He concludes by saying that if one is unable to prevail upon one's community to laten the schedule one need not abandon them, and would fulfill his obligations by joining them. He does not explain how/why. But it might be on the basis of such separation from the community constituting undue arrogance, which he alluded earlier.
וכתב המרדכי ובהג"ה במיימון וראבי"ה כתב דדברי רבינו תם עיקר הם, והבא להחמיר על עצמו ולהמתין עד [צאת הכוכבים] כדברי שאר הגאונים, מחזי כיוהרא ונקרא הדיוט אם לא הורגל בשאר פרישות.
This would go as far as explaining his recommendation for one who is already embroiled, but not justifying the practice to begin with.
