What about the cost of those special collaboration rooms that no one wants to talk about?

At my previous cube farm employer, we had maybe 75 cubes, a giant 30x30 cafeteria/lunch/meeting room, oh 16 tables to eat lunch at least, a large conference room we literally called the large conference room of 20x20 and a small conf room we called the small conference room of 10x10 and there was an engineering team meeting operations room (really a lockable storage room / lab) that was 20x20. Because coats and boots "can't be stored in cubes" although we did anyway, there was a row of 50 feet by 3 feet of coat closet that was basically unused. That's a lot of square footage allocated to no individual therefore "saved" but offices would result in 2000 or so sq ft of shared space being eliminated. Now figure a 10x10 office shared by two people, thats 40 people's private offices just being wasted in the shared space required by cubicle life. So of the 75 people in that office 40 are in the new offices and 35 are distributed in the space formerly occupied by cubes. Certainly cube walls are slightly thinner than private office walls but the space savings won't be a factor of two. Definitely the employer was throwing away a considerable amount of expensive rent by using cubes and meeting rooms instead of private offices. If they junked the cubes and went private shared office they would have still had extra leftover space maybe for fancier larger offices or some people could have solitary private offices or maybe some "neutral ground" meeting rooms.

This depends entirely on the employer and what they provide for in the office.

Yes, if they provide special "collaboration rooms", that's going to add to the cost. But if they don't, then it's not a factor. If your employer just gives you one big open room with a bunch of tables, and that's it, that really doesn't cost much. And there's a bunch of employers these days that do exactly this.

(As for coats and boots, you can put your coat on the back of your chair. Or drop it on the floor under your desk. Yeah, it sucks, but again there's plenty of employers that treat engineers this way these days.)

The open office is terrible for having meetings, calls and conferences. All the companies I've seen had to allocate about half the building space to meeting rooms only. That totally ruins the alleged space savings from having a single cheap hangar full of people.

There wouldn't be need for that many rooms if there were proper offices.

I don't disagree, but I'll also point out that it isn't much better with cubicles. When I worked in a giant company that had cubicles in the 2000s, it was the same: a significant part (not half though) of the floor space was dedicated to meeting rooms, because you can't have meetings in a cubicle easily, and for privacy, serious discussion, etc., you really need a meeting room. So a company like that, which is already set up with the 1990s standard of cubicles and meeting rooms, could easily see an open-office plan as a way to save money and pack even more people into the office buildings they already have. All they have to do is take out the cubicles and stick a bunch of tables in, and leave the meeting rooms as-is.

But yes, if they had proper offices, they wouldn't need many of those meeting rooms, only some larger ones for meetings that are too big for the offices (more than 3-4 people perhaps).