Why? Maybe for a similar reason you've misused the word incredulous*
(describes a person that is expressing incredulity not a thing about which they are expressing incredulity): we don't pause to weigh and consider every syllable we utter, we create sentences at speed to convey meaning.
We say 'p' because we see it on the page (and hear others using it) and don't stop to consider whether we need to unpack the abbreviation, because it's a common usage and our listener will understand, so why not?
Actually, I think "pence" does take more effort to say than "pee" because you've got to close off the vowel with the "s" sound.
*I know that sounds like a harsh dig, but actually, we all make errors in our language, and I think we should be pretty forgiving. It was clear what you meant by your use of "incredulous", so I wouldn't normally let it bother me. But this is Pedants' Corner!