You can see the reason for yourself with a simple experiment!
-Install a Linux disro on your machine (i.e: Ubuntu);
-Write a simple C program that does a long-ish calculation, like sorting 10000 numbers.
-Set the program to use an RT scheduling policy: https://man7.org/linux/man-pages/man2/sched_setscheduler.2.html
- Use a utility such as perf to measure execution time, run the program as many times as possible and record the runtime for each try.
- Is the timing consistent? Are there some runtimes that are reeeeealy long compared to the others?
Should give you an idea of the problems with using Linux in certain ES applications. Of course, in many cases Linux is a perfectly good RTOS
I think these kinds of questions are mostly academic, sometimes asked by professors (as part of a university course) who have no ideas or practical experience in the industry. What variant of Linux is the question talking about? How do you define a real-time operating system? If it's all kinds of Linux in this world, then the statement is certainly not true at all. Have a look at uClinux (**broken link removed**)