It originated because we know for a fact that lab leaks do happen. SARS leaked from a lab in Beijing a few years ago and there have been leaks in Russia, the USA, the UK and many other countries. It's just a fact that these things can happen. That by itself doesn't make this particular lab leak theory particularly likely or plausible, but it definitely make it possible.

I have no time whatsoever for the nutters who say the lab leak is definitely correct or has been proven. Likewise I have no time for people who say the lab leak was not possible and should not be considered. In both cases there's a big chunk of motivated reasoning, and quite possibly wilful ignorance going on IMHO.