It messes up real-time gameplay. If they did that, they'd have other NPCs birthing children and NPCs aging and dying.
You're being unreasonable. Coding is way more complicated than you seems to understand.
If Beth was going to make thousands of NPC have children, grow up, age and eventually die of natural causes, you'd have decades of work before you with technology not available for public use.
Sure, they can randomise NPC, but that removes half of the point of TES games, and they would still have to code it all.
Wait, you're an old member around here. Sure you're not just trolling us?