Because they're lecturing about engineering efficiency, yet they require 2,000 engineers to support their product. By comparison, WhatsApp scaled to 3X as many active users with a engineering team 1/40th the size. Instagram also runs circles around them in this respect.
For a team so huge and so productive, where is the output? More productivity tools? Maybe Twitter should pivot into that space.
Can you enumerate Twitter's products?
Perhaps Instagram is a closer comparison -- broadcast photos instead of broadcast 140 characters. And you have a much smaller engineering team, much faster growth, twice as many active users, and far fewer fail whales.
Regardless of the comparisons, 2,000 engineers seems excessive in proportion to Twitter's product, which suggests engineering inefficiency.
Instagram has only recently started to monetize the product -- it's almost unfair to compare the size of the engineering teams supporting the systems.
Also, I suspect that their strategic plan is not about minimising the number of engineers. They may want to be able to set up non-public projects which iterate for years or months before becoming visible: frameworks, app ideas, you name it. Their Fabric platform gives them excellent insights into what people are doing on mobile apps, for example. The kind of insights and understanding they have into personal and aggregate app usage is quite frightening when you realise just how much they have instrumented.
However they're much simple products, subject to a lot less load
I think Instagram hasn't achieved the kind of load that Twitter has had a couple of years ago
Whatsapp has a higher bandwidth, however groups seem much simpler to manage (messages stay in small groups)
How many API requests happen every second for twitter and instagram?
Instagram has only began support for free format photos now.
So maybe twitter engineering is inflated, but it's a different product beyond the 'stuff in a timeline' description
Maybe that's the point!