I've written one anyway.
If I was building this site for a client this is not a decision I'd have made. There are numerous existing blog engines out there, they have more features and with their larger userbase they probably have less bugs. In pure economic terms blog engines are not something you do yourself unless you have a client who likes wasting money.
Fortunately for me this is my personal site and no one's going to fire me for reinventing the wheel. Which is nice because I like getting paid. The reasons for doing this myself are not unique.
- This is my testbed for various approaches. I'm going to learn more by doing this than by using an existing system.
- I tend to work in the business tier these days. It does me good to get closer to the shiny things.
- This is my personal site and I want it to work my way. Dropping in a third party system gives up a lot of control.
Software developers love to build everything ourselves. It's practically the only thing we have in common amongst our different factions. Building for myself I can write as much or as little as I like. When someone else is paying, their requirements take priority. And that's where it gets complicated.