Crappy plastic chairs, $5 non-brand keyboards and recycled CRT monitors are also cheaper.
If you want results from people working in a highly focus intensive and cognitive loaded job, and care about their mental health, adding divider walls is an irrelevant cost.
You already pay them 70K+ per year, it's not some 20K/year developing world call center.
You also don't need to give everyone their own office -- you could have people in smaller units of 2 to 4 persons.
This has the added benefit of ensuring that the $70K/year or whatever you're paying these people stays static. Experienced people have a bad habit of expecting to earn more money as their skills deepen. By keeping your workforce in a constant state of churn, you drive those people away before they can start making demands.
This is fine when it's 5, 10, 20 people but open offices of 200+ people that is a combination of engineering, product, sales, marketing, etc does not. Even when small groups have conversations it's suddenly magnified through the whole office.