I feel it's caused by two things:

• In industry, most people do more reading than writing, so you see a lot of other people's mistakes and have to fix them rather than your own. You don't make enough code to feel humbled.
• Out of industry, there's often a vacuum. You code one way and make a thing and you're proud of it. You never hear criticism, and you're defensive of your abilities. This could be programmers who are new or just out of college or do it as a side to their main job. You don't share code enough with people to learn better ways and be humbled. Good enough is enough to be proud of.

There's an in between state that can open up the door to humility. Maybe a person who works at a company and thus deals with customers, non-programmers, and a team but still works on open source and in their free time build lots of side-projects and open sources them. You're making enough code and putting it out there enough to really receive good criticism. Those people would be more likely to be humble I suppose