A replay makes it easier to verify a run wasn't cheated. Why isn't it mandatory?

We are of course aware of this possibility. It would be good for verification, true. But the community has so far chosen not to require it, as it is too easy to forget to enable it, and it would lead to many rejected otherwise good submissions. Especially since the enable replay button defaults to "off", and it reset to "off" by Factorio even if you previously enabled it (Unless you use the secret setting in "the rest" hidden settings, but too many people do not know that).

Adding additional requirements to submissions is a balancing act between verification and user friendliness. I think the reasoning for not requiring replay is that cheating is probably not a big enough problem in this community to add cumbersome requirements. The community runs on trust.

The requirements for submissions are subject to change, and are voted on by people who have submitted speedruns to the leaderboard, via the discord.