All of these companies that fund Linux development make lots of money from Linux, and that's why they fund Linux. Even as desktop Linux is sometimes a competitor to Microsoft Windows, Microslop makes way more money from their enterprise software which runs on Linux, so does Amazon.

And because Linux is GPL, they cannot just take the code and spin their own version and sell it to customers without also making the code GPL, so they necessarily have to contribute to the Linux kernel if they want to also use it. They are forced to make it better, they are forced to pay up.

In cases where the project does not use a GPL license, (for example, FreeBSD, which uses the BSD license), companies just rip them off. An example is Sony, whose playstations run FreeBSD based operating systems, but Sony rarely ever contributes or funds FreeBSD development in return. This is because the BSD license allows them to take the code and make it proprietary and sell it for money themselves. With GPL, this would be illegal.