It used to be that each runner was allowed a false start but that later changed. When it changed, a runner could false start and remain but that meant the next one to false start would be disqualified. Now a false start means immediate disqualification.
I can understand it's frustrating but imagine if there's eight runners, all twitching, there could have been eight false starts and no-one would have been disqualified. If you had several heats in major events they could have gone on for ages if there were multiple false starts.