That said, a common reason for it is back-door age discrimination. Plenty of companies use open-plan offices without such an intent (they "look" busy, which pays off in marketing, and they're cheap) but the open-plan fetishism is all about "culture fit", meaning exclusion of the old, the female, the lower-than-upper-middle-class, and all disabilities except early stage alcoholism.
Tech companies are, in general, very badly managed. It doesn't prevent them from being successful because technology is so powerful that it compensates for inept management. I see this as a good thing in the long term (imagine what this industry could do if it wasn't run by idiots) but, culturally, it's bad because the worst attributes of "startup culture" are infecting the whole economy.
Wait, what? How do open office plans put women, older folks, and lower-income folks at a disadvantage?
It doesn't come naturally to people whose life experiences involve violations of trust, embarrassing illnesses, or physical danger. (With age, the percentage of people who have such experiences increases, and approaches 100%.) You have to still have that sense of immortality to be OK in an open-plan office.
Also, having children makes people less trusting of strangers, for obvious biological reasons. Having a helpless being that you must protect at all costs makes people less OK with the blind trust in strangers you're supposed to have if you want to fit in to the "I'm at one with everyone, man!" California culture.
Older people are also not able to take as much abuse of their fight-or-flight systems, and pregnancy has a similar effect on women.