Because they want to have a tighter control of user experience, which includes the choice of the font used to display text. So they list their preferred fonts first, with fallback fonts at the end.
It’s a fair reason with an appropriate CSS pattern.
I, like the GP, fail to see how listing dozens of fonts of completely different nature (x-height, line-height, width, proportions, weight...) provides any "control of the user experience".
This seems to be developed for browsers which do not default to system-wide fonts: which ones are those today?
It’s a fair reason with an appropriate CSS pattern.