Because it is a very special device: It provides features that no purchased device offers.
The detailed answer: The overhead alarm clock offers the following features
Time and alarm time can be read effortlessly and glare-free even in the dark; without glasses, without pressing buttons, without having to leave the right or left side position.
The alarm clock can also be operated in the dark and with only one hand.
The alarm clock can be used by a first-time user by looking at the control panel. Reading any operating instructions is not necessary.
It wakes you up with a pleasant, volume adjustable sound (MP3 song).
It also functions reliably in the event of a power failure.
It is very accurate and independent of the reception of a radio signal, the power line frequency and the ambient temperature.
It does not occupy space on the nightstand.
How to achieve these qualities?
The device is screwed to the wall above the headboard of the bed.
The housing, the display and operating elements are particularly large.
All elements except the volume control are located on the front panel.
The marking (text or symbols) is large and rich in contrast (black on white).
Procedures like "Press this for 3 seconds and then 2x that..." do not exist.
The two LED displays permanently show the time (red) and the alarm time (yellow), so there is no doubt even at night whether and for what time the alarm is set. They are individually
dimmable. In addition, there is a tinted film in front of them, so that you are not disturbed or even dazzled even in absolute darkness.
The rotary switch switches to the setting mode for clock or alarm time. The hours and minutes can then be adjusted using the plus and minus buttons.
Unwanted alteration or deactivation of the alarm due to involuntary arm or hand movements during sleep is excluded by the use of rotary switches.
The (replaceable) MP3 alarm file is stored on an SD card and its volume can be set by a potentiometer.
A snooze function is intentionally not included, as the sleep time can be set easily and in a flash to any later wake-up time.
A USB battery pack is used for power supply. Normally it is charged and delivers the 5 volts needed by the Arduino at the same time. In case of a power failure it takes over the supply
(during my tests for several days).
The real-time clock module is quartz-controlled and also has a temperature sensor that corrects the clock speed depending on the ambient temperature. This results in a very high accuracy.
Even if the 5-volt supply from the Arduino is down, the clock module continues to run with a 3-volt button cell.