I'd say that one unmentioned reason for telling the truth is that the world is a complicated place. As such, it's impossible to fit all of it in your head at once, and it's impossible to constantly keep close tabs on everything going on around you. Therefore, it is essential to your sanity and ability to function that you can trust that certain things are as they seem without having to constantly verify that for yourself.

It's a tragedy of the commons thing: if everyone tells the truth, we can all trust each other and get on with doing interesting stuff. But if someone lies, they can abuse said trust to acheive their own ends that not everyone may agree on. And this comes at little cost to them; the external cost of them lying is that everyone else must expend more effort double-checking others and thus less effort doing interesting stuff.

Science (not the practice but the philosophy) depends on this: we assume that things in reality hold even if we aren't observing them right this instant. Sure, observations may affect reality, but it's not like the particle-wave duality of light stops existing if nobody's running a two-slit experiment.

Ultimately, lies prevent us from being able to effectively work towards change.