My money would be on somebody STUPID being in charge.
There is RARELY a legitimate reason to inline scripting... the only REAL reason is to set values the external script might use.
If it's a page that has anything OTHER than bounce for traffic or a page that gets more than one view, it's a missed caching opportunity. Nothing more, nothing less. Same goes for 99.99% of the time you see style="" used and 100% of the time you see <style>. It's more a matter of developer ineptitude, ignorance, and incompetence than it is much of anything else.
... and yes, massive fixtures of the Internet CAN end up with people that STUPID in charge of things like that. Just because "oh it's instagram" doesn't make it herp that derp any less. SOMEBODY went full Pakled.