It's unclear if the planes mentioned in the OP are talking about Amazon actually outright purchasing planes (which would be a first) or if they are just leasing more.
edit: Actually the article does say this at the bottom:
>Amazon launched its own air cargo fleet in 2016 and, prior to Tuesday's news, the company leased 80 planes, but the move is the first time the company has bought their own
These were probably not leased because they were very cheap and had relatively little future value, making them unattractive for leasing companies.
https://www.bloomberg.com/news/articles/2021-01-05/amazon-ma...
I believe as of late Amazon has been dry leasing aircraft and contracting with Air Transport International, Atlas, Sun Country, and others for crew and potentially maintenance as well.
[1] https://www.aopa.org/news-and-media/all-news/2020/march/pilo...
Airlines are more likely to buy packages of service from manufacturers (especially engines) and big service providers if they don't have enough scale or experience to do their own maintenance, regardless of how it was paid for.