It gets dark early because of how Earth is tilted and how it moves around the Sun. This changes how long the Sun is above your horizon at different times of year.
Earth’s tilt
Earth’s axis is tilted about 23.5 degrees instead of standing straight up, so during part of the year your half of the planet is tilted away from the Sun. When you are tilted away, the Sun takes a shorter, lower path across the sky, so it rises later and sets earlier, giving shorter days and longer nights.
Seasons and daylight
This tilt is what creates the seasons and the big difference in daylight between summer and winter. In winter, the Sun’s light hits your area at a shallower angle and for fewer hours, which makes it both colder and darker earlier in the day.
Clock changes and perception
In many places, changing the clocks for daylight saving time also makes it seem like it suddenly gets dark much earlier in autumn. Cloudy weather and the faster day‑length changes in fall can make the shift feel even more abrupt, so evenings feel “too dark” very quickly.