The reason why we hear "pretty sure" or other synonymous phrases ("I think", "I guess", "I reckon", "I'd wager", "probably", "maybe", "sorta", and such like) is because it is no longer acceptable to be confident in yourself and what you know. Paradoxically, it is no longer acceptable to admit you don't know. Both "I am confident that..." and "I don't know" are "trigger" phrases nowadays. Both of these phrases have been attached to "trigger" categories of our day: "confidence" is attached to "toxic" masculinity and "I don't know" has always been associated with being dumb. Neither of these statements in and of themselves exhibit the categories they've been shoehorned into.
OK, I'll get off my soapbox.
Moral of the story: be more confident and speak confidently.
I am working on that for myself and have found it takes focus in order to carefully avoid saying passe phrases like "pretty sure" and "probably".