Even the ones who do understand a little about the checks probably don't bother thinking about the difference between "sent" and "delivered". They'd understand it if it was pointed out to them, they aren't stupid. But they don't care enough to realize it because they shouldn't _need_ to understand it most of the time.

And even so, the checkmarks are very subtle and easy to not notice if you don't expect to need to look at them. A user is more likely to say "well it didn't give me an error so it must have sent, I wonder why nindalf is ghosting me" rather than "huh, I wonder if WhatsApp actually _delivered_ the message to nindalf, let me check"

I use it somewhat reluctantly which might reduce the degree to which I actively seek out understanding. I wish we'd all go back to vendor neutral channels of communication but I also apprecitate the fact that it is less sucky than SMS.