Amazon also spends a lot of time and effort on Open Source software, including the Linux kernel. They also contribute money to various open source foundations and target funding for various groups like OpenSSL.

Microsoft gets all the news because it's a drastic change from normal behaviour.

Amazon, Google, Oracle et all just keep steadily contributing stuff. A lot of the work is also deadly boring and not headline capturing stuff.

Few simple queries to show the work they're doing just on the kernel:

Amazon: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/lin...

Google: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/lin...

Microsoft: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/lin...

Oracle: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/lin...