And it is true that this is the way many people use whiteboards, as a kind of presentation tool rather than a thinking tool. But that is largely down to lack of practice with the chalkboard/whiteboard as a personal, one-on-one, or small group tool.
There are plenty of people who successfully use white boards as a tool for routine technical conversations in small groups, with multiple participants writing on the same board.
“Not everyone is writing literally simultaneously” would be a vacuous and uninteresting statement, so I’ll give the previous poster the benefit of the doubt that he/she was saying something more substantial. No kind of conversation involves every interlocutor literally simultaneously talking/writing in the same space at the same time: that would just be a cacophony of monologues, not a conversation at all.
What this means in practice is that we have a lot of software that makes it easy to bring a group of strangers in to pseudonoymously vandalize a whiteboard by drawing dicks on it - a true "cacophony of monologues." See also: most comment threads.
That doesn’t mean people collaborating won’t be taking turns as a practical matter, enforced by social convention.
Chalkboards are actually a great way to communicate and collaborate in small groups, and have been for a long time one of the main forms of communication in professional mathematics and some other technical fields.
The chalkboard is a very democratic medium, because anyone can more-or-less produce the same output, it is easy to add notes over / adjacent to other people’s writing (including formulas, diagrams, ...), and the possibilities are very free and open-ended. This stands in contrast to fancy typography and bespoke technical diagrams, which are often slow to produce, difficult to modify, and require great expertise to do well.
Using a chalkboard well as a collaborative thinking tool takes practice though.