With all these games (this one included), I've been wondering why world record times are faster in single player. Is there an exploited glitch in all of them that doesn't work when there are just two playable characters and both are consistently controlled by human players? Yes, I know at least some of these games allow one player to use two controllers, but I doubt one person controls two characters throughout the whole run.

co-op runs take a lot more coordination than solo runs since they require two players to be available for the length of the run... and willing to do the run with the other person

All LEGO games allow 1 person to control both players, and top LEGO runners use this pretty much wherever it saves time. So there really is not that big of an advantage from that perspective to playing co-op instead of playing solo. In fact, there is definitely a bit of a disadvantage to co-op unless two players practice together and coordinate their strats. Most co-op runs are more of a "for fun" thing that two runners do rather than a serious attempt at grinding a competitive time.

A noteworthy example of co-op runs where the two players prep and practice quite a bit can be seen in Shred and Jarool's runs in The Skywalker Saga, which are way faster than the solo runs.