It has been my observation that the people who are the most vocal about animals not having sentience are the people who have a strong opinion about human-centrist ideals, with an emphasis on "humans first".
this is not meant to be an attack, more that there seems to be a synergistic bias that re-enforces in both directions with such people.
They also tend to not interact meaningfully with animals on any kind of regularity.
Conversely, you often find radical notions of "Animal Personhood" in people with the opposite bend in their personality (see for instance, the more rabid members of PETA.)
I approach it from the perspective of somebody that grew up out in the boonies, who interacted with a large variety of animals regularly. "No shit." is my reaction to the news of "
Scientists discover animals have internal worlds and are aware of the present and future" rhetoric. I've known that for years. Are they 'people'? No. They do not have the impulses or interests of people. They do not, and will not, act like people. Are they aware of their situations? Yes. Do they have emotions and form emotional attachments with each other (and people)? Obviously-- look at the separation anxiety dogs have. Of course they do. Are they aware of causes and effects? Absolutely-- Pavlov's dogs being able to be classically conditioned is empirical proof of this, and it has been known for almost a century now.
Some wild animals even exhibit indications that they have theory of mind for other creatures that are not like themselves-- For instance, squirrels and crows will actively watch other living things for eye contact, and
will engage in deceptive behavior when storing/caching foodstuffs, or collecting interesting objects. They are very much concerned that you are interested in what they are doing, with emphasis that they are worried you will steal their nuts/shinies.
As such, my reaction is "And science is only just now starting to actively realize this, ---- because?"
My only explanation, is that there are such systemic biases toward human-centrism hard at work, preventing the long laundry list of findings stretching back over a century, from sinking in. Mostly because accepting that reality, would mean changes in habits of personal consumption and behavior that are "unwanted."