Twitter hasn't focused on anything else besides twitter. Facebook is taking on the mission of "connecting the world". Google has gone well beyond being only a search engine. Amazon has expanded into mobile, AWS and ... Twitter, on the other hand, is still pushing the same product.
Taking a step back, what's the advantage to society of every company growing into a conglomerate? Why not just let a company live and die on its core business? Do we really want to allow all that power to be consolidated into company boards and executives?
Companies will want to continue exist and generate more profits for their owners, which means deploying new products - and these products might appear to be different but upon closer inspection they kind of make sense. Facebook is connecting everything (core product + whatsapp + the internet balloons etc.). Googles core business appears to be analyzing large, dynamic and complex datasets (the web, maps, their biotech research, self-driving cars).
Of course there are always outliers (Android), but companies go for profit. If they can incorporate something new they will.