- Steve and Alex founded reddit fresh out of school, and schools don't generally teach databases beyond the theory
- Fresh out of school, they didn't have the opportunity to learn it from someone with more experience
- They were busy building the rest of the site, and so didn't spend the time to delve in to these concepts
- Paul either didn't feel the need to explain it to them, or had reasons similar to the above to not know
Hmm... I'm out of ideas. Anyone else?