I'd wager at least part of it is the same reason many companies use CloudFront: Separate point of failure in case of downtime at the AWS side, some one else shouldering DDoS.
Pure speculation, but probably the same reason they ran Oracle for so long -- not many cdn providers when amazon.com launched. Afaik akamai still has one of the biggest edge networks of all the providers.
Re:What happens when you update your DNS
by MaxBarraclough on Tuesday June 23, 2020 @05:58AM
(#23611844)
I asked the same question 7 days ago: https://news.ycombinator.com/item?id=23537135
(My first thought was, wow, surprised they outsource that.)