It's a fair question.
Data centers are wildly expensive to operate if you want proper security, redundancy, reliability, recoverability, bandwidth, scale elasticity, etc.
And when I say security, I'm not just talking about software level security, but literal armed guards are needed at the scale of a company like Figma.
Bandwidth at that scale means literally negotiating to buy up enough direct fiber and verifying the routes that fiber takes between data centers.
At one of the companies I worked at, it was not uncommon to lose data center connectivity because a farmer's tractor cut a major fiber line we relied on.
Scalability might include tracking square footage available for new racks in physical buildings.
As long as your company is profitable, at anything but Facebook like scale, it may not be worth the trouble to try to run your own data center.
Even if the cloud doesn't save money, it saves mental energy and focus.