Most game companies are a tiny fraction of that size. Even most AAA games are made by teams of hundreds. Not teams of thousands.
They also have their own Steam competitor (Epic Games Store) and, more importantly, they develop and support Unreal Engine used by tons of other game dev companies.
If you want an apples to apples comparison (i.e., other big live-service game companies) in terms of the employee count, you got:
Mihoyo (Genshin Impact, Honkai Star Rail) - ~5,000-6,000
Riot Games (League of Legends, Valorant) - 4,500
Roblox - 3,500
I know they're sponsoring a bunch of ARM and Linux projects as well.
Having skilled and happy employees that aren't constantly changing and do not spend all of their time on ways to fuck over customers and chase trends is simply impossible. Releasing a piece of hardware and leaving it open for customers to do with what they want? Linux? Not hiring people the second line goes up and then immediately firing them when line stagnates? Preposterous.
Mihoyo literally prints money with predatory gacha
Riot has had several layoffs in recent years
Roblox loses tons of money every year
Though Unreal Engine does indeed need quite a few developers. Additionally, using UE is much cheaper (5% on games exceeding 1 milion USD gross revenue) than using Steam (30% on every game). So they not only need more developers than Valve, they also earn less money.
(Not saying this is justified, of course. I think Unity is pretty much doomed.)
That globalization is a big reason many tech companies swell. When you need a team to work in and around every region's laws and regulations, you get big quickly.
But also, unity has slimmed down and scaled down on a lot of initiatives.
