They didn't say lab produced. The prevailing theory among those that are suspicious of the Wuhan lab is that it was a natural virus being studied in a lab from which it escaped.

Also, Occams Razor has nothing to do with 'who gained' https://en.wikipedia.org/wiki/Occam%27s_razor

Not lab produced. Simply lab leaked. There seems to be a conflation between "covid was cooked up in the Wuhan virology lab" and "covid was naturally occurring in bats being examined in the Wuhan virology lab, which ended up being ground zero for the pandemic due to poor containment practices." In this scenario, no one "gained from the virus" because releasing the virus was unintentional.

There's little hard evidence for the lab-leak theory, but there's little hard evidence for any other theory as well. There is however, a large amount of circumstantial evidence that makes the lab-leak theory more compelling.

    - The earliest known cases have no connection to the wet market, meaning the wet market likely was not where a crossover event would have occurred. We've known this for nearly a year now.

    - No actual intermediate host has been found, to this day.

    - The closest naturally occurring coronavirus that has been discovered in the wild occurs in a type of bat that is thousands of kilometers away from Wuhan. However, this type of bat is among those being studied in the Wuhan Virology lab.

China has been extremely uncooperative in efforts to discover the origins of the outbreak:

    - In the early days of the pandemic, after shutting down the wet market in question, they euthanized and sterilized the entirety of the market over the objections of international community, effectively making it impossible to know what intermediate host there might have been (though this was later made less important, given that earlier cases with no connection to the wet market were discovered)

    - They resisted allowing international investigators into the country to probe for the origins of the virus for nearly a year. WHO investigators, who were only allowed into the country in recent months, have concluded that the virus likely came from wild animals and not the lab, but in the same breath says their report is inconclusive because they weren't actually much to work with by the Chinese governemnt. Additionally, questions have been raised about raw data regarding several hundred of the earliest known infections either going "missing" or simply being withheld by the CCP. 13 countries, including the US, today have basically called the report a joke given how little data they were given to work with

    - US intelligence agencies have claimed to have some information backing the theory of a lab leak. This could simply be sabre-rattling, but its worth noting.

    - Labs in the US stopped performing similar research to the one in Wuhan have stopped in the last decade or so specifically due to concerns over scenarios just like the current pandemic. On the other hand, the Wuhan lab in question was specifically called out for lacking the proper personnel and safety containment protocols for this type of research by international inspectors going back several years ago.

- Experts like former CDC Director Robert Redfield, who have worked extensively in virology, have gone on the record to say that the lab-leak theory is in their opinion the most likely given what we know.

Meanwhile, many attempts to discredit the lab-leak theory generally fall into one of two camps:

    - People conflating "lab-leak" with "artificially created virus", which are not the same thing

    - Reports from the WHO, who has shown extreme reticence to do or say anything critical of the Chinese government

Literally none of the above is concrete evidence of the lab-leak theory actually being the case. It's still entirely plausible there was a spillover the same way there has been with many other viruses in the past. But in total, all of the circumstantial evidence, combined with the lack of any real evidence regarding spillover theories, make it at least as likely as anything else we have.