First of all, early Shabbos is a Kiyum Asei D’Oraisa according to almost everyone except the Rambam.
Second of all, DY beat me to the Rashi that HKBH doesn’t have Tosefes Shabbos because He can be Metzamtzem while we can’t (thus, Tosefes Shabbos would actually take away from His perfection and thus it’s not Shayach to Him at all).
Third of all, many places on Earth are just not Shayach to take in Shabbos late in the summer (try London). I know big Rabbonim who are not Makpid on the Mishnah B’rurah’s Chumra to have a K’zayis of bread after Tzeis Friday night because they are asleep before then, as Tzeis can get close to or after midnight (especially if you want to hold 72 or 90 minutes or even degrees). So people fall back to Ikkar HaDin (that early Shabbos works) because there really is no choice.