I’ve always found that assumption very weird and figure it’s rooted in human exceptionalism. Like we must be super special somehow. The more natural assumption to me seems that other animals, given their similar biological makeup, think rather similarly to humans.
That just depends on what you consider the default state to be. Claiming that humans have self awareness, but other animals do not, implies a relationship between species and capability for self awareness. The null hypothesis would imply a lack thereof.
It would be correct and good to acknowledge that we simply don’t know whether a given species is self-aware unless evidence points to one or the other direction. And that is very relevant for moral philosophy.