We've recently moved into new offices in Frankfurt/Germany and being the CTO of our 40 Person Startup gave me an in-depth look at all the calculations involved. It's simply horrible how expensive it would be to give private offices even to >3 people in our company.
I've fought tooth and nail to keep everyone in small team offices(We've got two small rooms set aside as flex-desks(complete with 2 4K Monitors & Gigabit Ethernet via a TB3 Dock) to give everyone the possibility to work in a 1 person office when its critical, but that combined with our lax home-office rules is more of a best we can do approach than a real solution to the office plan/distraction problem.
The problem stems from there being simply no officespaces available that have a default layout that permits many small rooms while retaining quick access to group/meeting and social areas. This seems to be a result of different design goals of other professions that need to maximise for the most amount of butts in seat per square meter, without factoring in the losses that distractions can cause.
If you want to redraw the floorplan, you have to sign rental contracts upwards of 5 years, something no sensible startup would do and even if, the whole investment is relatively large and needs a lot of focus from the company management to ensure its worthwhile. The expensive part is not the additional room the company would need to rent for each employee, it's the amount of empty space(hallways and large rooms) a company would need to rent to have more small rooms available up front.