Why are vest tops not a thing in cycling?
Triathletes seem to get the option, or at least some kind of tank top style thing. Runners get proper vest tops.
“Cyclists” seem to shun vest tops, mocking others they see riding in them. There doesnt even seem to be any options for sleeveless cycling gear.
Genuinely intrigued as to why that is?
Sun protection or warmer when the suns not out, aerodynamics, or if you use a backpack it protects your shoulders too are a few reason’s I can think of.
Nobody wants to look like a Triathlete 
Kidding - but a lot is fashion and history. Pretty sure USA Cycling and maybe UCI have rules that mandate sleeves in addition to the good points HLaB makes.
Tradition. Cycling is a relatively old sport, and showing that much skin was not appropriate back then.
Also northern Europe is, and especially was, often not warm enough to think about going sleeveless.
Here in the US, for years sleeveless shirts were viewed as being déclassé and we had a number of pejorative names for them. IDK if this has any bearing on their use in cycling.
You could further specify this with a ‘for men’ restrictor. I see plenty of sleeveless tops for women. I’ll also add that I did have some male friends who rode in sleeveless tops many years ago, and I truly hated seeing their pit hair poking out.
I wear UPF long sleeves or arm covers most of the time in the heat - cooler and I don’t get roasted by the sun. Why would I want to wear a tank top that makes it worse?
Because they are naff
So is that a hard no on a topless half vest, sleeveless jersey and white arm warmers with ankle socks? I would like to see Rapha Pro Team Aero jerseys with front chest pockets- for real.
rule #7
Funnily enough, on first glance, I thought this was a woman. Thats how ingrained sleeves are for male cyclists.
A pictorial example of why you shouldn’t.
Cyclists only transition from bike to café and if a cyclist is waving their arms about, things have gone wrong, thus extra freedom of movement or using with a wetsuit are not relevant.
The picture posted above is a better answer tbf.
Arthur A. Zimmerman wearing his typical racing jersey, bearing the logo of the New York Athletic Club, of which his father was a member. Image credit: Monmouth County Historical Association.
He was a three-time U.S. champion, in 1890, 1891, and 1892. In 1893, he was the first-ever world cycling champion. During this time, and for most of his adult life, he lived at 1106 Fifth Avenue in Asbury Park, with his wife, C. Blanche Zimmerman, and their daughters Ella and Hortense.
from: Arthur A. Zimmerman: Monmouth County’s World Champion Cyclist | Monmouth Timeline
If you look hard, you can find others. Won’t change anyone’s mind, though. I’m in the “I have old man skin” long sleeve crew pretty much all the time these days.
SA
A pictorial example of why he shouldn’t. 
Correct, USA Cycling rules state that jerseys have to “cover the shoulders”.
He also wears his bib over the jersey from time to time, not the best fashion reference
