The "office empires" make perfect sense at the size of FAANGs. When you need to hire that many people every year, many of them who have never worked in "the real world" before, how do you do the best to ensure they all have access to knowledge, resources, support, and oversight? Not everyone is good at self directed work from home. That doesn't mean they can't be an outstanding employee. Hiring is expensive and if you can avoid having major churn in employees you'll save quite a bit of money.

On top of that, being a major employer in a city gives your company sizable clout when it comes to lobbying the local government. Don't like the new tax plan? It will probably work out in your favor to let the city know that it would really hurt hiring and salaries at the biggest employer in the city. Heck if it hurts enough, you might even have to move to that nearby competitor city who is offering all those sweet tax breaks. You don't want to but it's the only responsible thing to do for all of your employees. It would be a shame to see a bunch of those employees leave the city and cause housing prices to drop for all the rest of the citizens. What's that? You're reconsidering that tax bill? I knew you'd make the right choice for your constituents.