Before revision: The real time clock is omitted either out of the desire to save money (as in the Raspberry Pi system architecture before Raspberry Pi 5) or because real time clocks may not be needed at all (as in the Arduino system architectureHowever, note that many Arduino software suites include an optional software RTC run from a low precision timer as described.)
After revision: The real time clock is omitted either out of the desire to save money