I would actually use this line at the beginning of a rant to the opposite effect: why can't people be professional in the workplace? Why can't they just do their jobs and go home, and not treat the office like a social club / bar?
[0] https://techcrunch.com/2017/05/15/uploadvr-sued-over-rampant...
My point is there is no common ground here. What is/isn't acceptable to me clearly differs from you. It will be thousands of years before society has any chance of finding common ground on what is/isn't offensive. What do we do in the mean time? Set arbitrary standards, or learn to live without them?
If you read actual examples it's pretty clear what people object to. It's not even that these are actual attempts to flirt or anything like that. Telling someone that you have an erection and concentrate because of her isn't flirting. It's a passive-agressive attempt to demean and hurt the other person.
First and foremost, it would make at least a heavy minority if not a majority of people rather unhappy. Because of a wagon-load of reasons (excessive work hours, moving to a new city for work separating you from most of your pre-existing social network, general difficulty of making new friends as an adult), work friends are the core of most people's social networks nowadays. If you eliminate work friends, you'd leave a lot of us more-or-less friendless, isolated, and sad.
It also becomes complicated when you consider that some work inherently involves discussing your personal opinions. What business practices are ethical, aesthetics of design, editorial choices.
Finally, employees must be able to discuss matters of common interest directly, to organize against management in both traditional matters of worker interest (conditions, pay, benefits) and to express a collective opinion about the direction of the company (eg, when the company takes actions the employees believe are unethical). This also requires employees be able to express potentially objectionable opinions to each other.
But as a model of professional behavior, it would make everything so much simpler if you never said one extra word to or treated the person in the next desk/cube/office in the slightest way different, inside the workplace, regardless of whether they're your best friend, your spouse, your kidney donor, your childhood bully, your crush, your worst enemy, or your closest relative. If you work with your best friend, finish your work, leave the office, and then talk to them about sports or your porn collections.