A very small startup team does benefit from an open plan, such as shared desks or a shared office. This scales to maybe 6 people. I think this is the environment that larger companies are trying to capture.
But at scale, this is a ridiculous setup and one that breeds interruptions. In a small team, what one person knows is often useful to another. In a larger team there are many more independent parts, and entangling communication is a waste of attention. Worse, I think it causes people to deliberately adopt a default-closed mentality to avoid overload.
Companies don't easily outgrow their early cultures; they think what got them there will continue to serve them -- in fact, they want to go back and "capture the magic" that were there in the early days. It's a form of corporate nostalgia that is one symptom of becoming sclerotic.
It's not really about open floor plan vs. cubicles. I recall the first time I visited HP's vast cube farm at their silicon valley headquarters. It was vast, drab, mostly quiet, and absolutely soul-deadening. OTOH the Dropbox headquarters in SF was open floor plan and absolutely cramped, yet buzzing with youthful energy. Lots of people were using headphones. I could see how it would be an exciting place for a young person. On the one hand the buzzing energy is a potential distractor, but it's also an energy booster. It makes your heart beat a little faster. It makes you feel like you're in a place where things are happening.
With age, my powers of concentration have waned a bit and I cannot power through distractions like I did when younger. A cube farm would be very demotivating. An open floor plan would be highly distracting, but likely somewhat better. But the best is to avoid large environments like that entirely, and stick with a smaller group where the interruptions are actually useful, where a shared office and a shared mission produce energy and focus.