Betting on speed runs or related games could be an interesting and fresh experience for gamblers. For instance, imagine betting on a Guinness Book of Records attempt, such as someone trying to beat the record for the longest cooking time.
Though, I believe you are going a little bit off the original concept of the thread/discussion, trying to beat an original guinness record is rather uninteresting market for bettors, in my opinion, because all the preparation it takes and all the time it takes for a single person to try to beat a guinness record. On the other hand, if you limit yourself to videogames and speedrunning, then it becomes more dynamic, it becomes faster and more appealing to bettors and gamblers in general. We all know how crucial it is for gamblers to be able to wager their money at a reasonable pace, not too low, otherwise they go away and seek for other alternatives which suit them the best.
Think for example of the original game Mario Bros for the Nintendo Entertainment system, the current world record is about 5 minutes. That means each attempt to beat the record would be about 5 minutes, before the next attempt, 5 minutes is rather a reasonable time for bettors. In contrast, there are world records (unrelated to videogames) which could take months to carry out.
There is much potential on it, and I believe it will be just matter of time before a gambling provider realizes it and start to sponsor.