Why does Windows require a restart after installing updates?

Wow, that's a lot of consequences for what probably seemed like a simple, obvious design decision. It's rare to see an honest acknowledgment of this, but it raises questions like "what other early, obvious design decisions make Windows goofy today?" Drive letters? Backslash as path separator? Magic file names like CON, LPT, AUX?

TL;DR: Unix inode indirection permits clean file replacement without overwriting existing copies or disrupting running processes. Windows' file system architecture lacks this concept. In-place upgrades aren't a realistic option for Windows without some serious contortions and caveats.