Google, Microsoft and Facebook have capital and compute. That is not an OpenAI moat.

Facebook has Moat because of their social network. It is very hard to switch to another network. Google with search has no moat because it is easy to switch to a new search engine. OpenAI has no moat because it is easy to switch to a new AI chat once a better product becomes available. AWS has moat because it is hard to switch cloud providers. Apple has moat because people want to buy apple products. etc.

A moat can be seen where even if you have a worse product than the competition, or users hate you, they still use your products because the cost to switch is immense.