Yawing seems like it must be adventurous, the contagious part not so much.

Even the mention of a yawn can trigger it.

Perhaps we are almost always in a state of needing a yawn, but the trigger is seldom met, and seeing or hearing about it is enough to make our brain go "oh yeah I forgot about that".

Perhaps yawning is actually underdeveloped and an ideal human would yawn at regular intervals without any prompting.

Not so much if you think about if from security point of view of our ancestors. Those 1-2s if we talk about proper yaw you are defenseless and clueless, its actually pretty dangerous during say high speed drive on tightly packed highway (as in every single car in all lanes goes too fast to handle any major driver's mistake). But its great for equalizing pressure in ear via eustach tube without the need to block & blow your nose, something both mountaineers/paragliders and divers are well aware of.

Same goes for sneezing, actually that's even worse for driving, I literally don't see anything for a second at least. Sometimes can be blocked, sometimes not so much.