What is the point of doubt in ultimate knowledge? Why believe that your senses might be lying to you?
I think the fundamental reason is because it shows the subjectiveness of the beliefs you hold.
Here’s the thing. Each person comes to a conclusion as to what constitutes proof for them. Proof is itself arbitrary: is seeing something happen proof? Does that then make hallucinations proof?
Now, my point is not that humans living on Earth cannot come to a general agreement as to what proof mostly is. The point is that whatever body of evidence that you claim is proof is necessarily based upon your opinion.
For example, let’s say you have an apple seed, plant it, tend it for five years, and watch it produce more apples. You eat the fruit and to you this is proof that it is an apple tree. The question is, if you had only tended the tree for four years, was that less proof? Would you be less convinced that it was an apple tree? It all comes down to your definitions, and the definitions themselves are arbitrary.
Another way to look at this is, what is a heap of rice kernels? Say I put down one kernel, and you say “No, not a heap.” I put down another, and another, and another, until you say “Yes, this is a heap!” Now, I take away one kernel, does it stop being a heap?
To know is to define something and be convinced of it. If the definition is itself arbitrary, and being convinced of it is arbitrary, then to exist without doubt is deluded.