- It's a smooth, regular surface without possible holes, rocks, and other things to trip on. It's also made for running, so it's not as hard as pavement.
- I generally run late at night. My living room is well-lit and free of two-legged critters who might do me harm.
- My living room is climate-controlled and well-lit.
- With a treadmill I have a predictable, repeatable experience, and both the controls and metrics for that are at my fingertips. As a corollary, the pace-change indications are in front of me. "Run until 17:30" is easy.
- Having small-bite progress markers in front of me keeps me going. An open trail thart didn't have very closely-spaced counters of some sort wouldn't work as well for me. I know that most people would be quite the opposite in this regard.
- My entertainment options are more varied, and don't require me to carry anything or wear headphones.
- If I injure myself, and I think this is a matter of when rather than if, help is more accessible.
The only downside I can see is that it requires a functioning treadmill. I can cause that to happen again with time and/or money, and likely not too much of either.
I don't imagine myself ever being a social or organized runner, so that's no issue.
Did I overlook anything?