Sports world is such a conservative place. They usually end up the way they started. Most of them never change the way they do things during their career.

Inventor of Fosburry flap says it took a whole generation of high jumpers to change the old technique, rolls on the frontside over the bar. The jumpers start their career rolling on the belly had never converted to flap method even though the result had shown quite obvious that the new technique was better.

Same thing you can say to the over sized racket tennis players use. Many expert were saying they were
for beginners until top pros start using it. The advantage was ovbious that they couldn't win against the players who used it.

Cycling world seems even more conservative. There is some kind of apprenticeship everybody has to obey the unwritten rules handed by the older generation to the next. One of the memorable is LeMond was criticized by the other riders for eating ice cream before the race. I never figured out what was the problem with the other guys but seems that was norm and known to every racers.
No mirror is just the way they started and they developed the techniques without it. They don't even think about if there is a better way. Just the same way, when a new driver learn how to drive without using mirror, he'll never get use to using it. He'll be in denial all his life, he's less aware what's going on around him than others. He thinks he can be just as safe as any driver who use the mirror. He always wonder why anybody put mirrors on his car in the first place.

Disadvantage of using mirrors on the bike:
None---Small bar end mirror made of Polycarbonate can be made at 20 grams. Aerodynamic resistance, not much.

Advantage of using mirrors:
* The rider can keep the head straight.
* The rider can keep watching on the other guys without taking eyes off the front. When everybody is sprinting, the fraction of a second becomes critical.

Disadvantage of no mirrors:
* You can't keep eyes on other guys. You have to take time to look around which means you take a look less often than using mirrors. You look back every two seconds, three seconds or maybe 5 seconds? either way, the guy behind you has an advantage of monitoring you continuously. 2 seconds is a long time. You can monitor the guy behind you continuously with the mirrors.
* Turning head create wind resistance. The best, aerodynamically is head forward face down, like you see the sprinters do at the finish line
* Turning head gives less leverage at the sprint. You don't see sprinters turn their head to GET leverage. If not for checking other guys, they wouldn't do it.

Advantage of no mirrors:
* Weight saving of 20 grams? Aerodynamics? You tell me.

Closest real world example would be, even though I wasn't there so I can only imagine, last year at the moutain stage of TDF. The stage ended with mild descent. The leader was in striking distance ahead of Armstrong and Landis. Armstrong gave Landis a go ahead sign to win the stage. Landis had run out of gas just before the finish line and Armstrong saw it. Armstrong step aside of Landis and sped past him and the leader. The leader(I forgot his name) kept look around made sure everybody in safe distance behind him. Between his turning heads, he missed Armstrong approaching him from other side. When the leader looked to Armstrong side, it was too late. Lance's wheel was already ahead of him. Probably everybody remember the picture of lance raising his arms, rolling the tongue in his mouth, wearing his yellow jersey. If the leader had kept monitoring where everybody was, he would have known Armstrong was sneaking up on him.

So, I explained to you what I think. Now it's your time to explain. If you feel like throw your emotion at me by some meaningless word, you'd better give the opportunity to someone else who can make a logical explanation.