1. The vast majority of creatures don't have opposable thumbs, that doesn't mean it wasn't evolutionary advantageous for humans to have opposable thumbs.

2. We have no way of knowing any of this because we don't even have a handle on what sentience is.

One assumption people seem to be making is that sentience is a binary: either one has it or one doesn't. It's possible that there are many levels of self-awareness, some which we would recognize and some which we would not. There may be levels of sentience humans have not yet experienced, even.

For instance, ants exhibit self-aware behavior when presented with a mirror test. Does this mean ants are sentient?

https://www.npr.org/2021/08/09/1026227683/mirror-mirror-on-t...

Evidence for this claim? We simply don't know that yet. Unless you are using "sentience" to mean "human" in which case, of course they're not. However, that's part of the problem - "sentience" is not necessarily unique to humans.