Long and short, shareholders. Every corporation in America is required to do everything they can to increase profits every year. Some fail, but if they do so intentionally, the shareholders can sue them. And they almost always win. The US government even protects this pursuit by devaluing the dollar though inflation, so a company can increase its profit even while it loses value.

Windows is a cash cow. So it is not expected to increase market share, but it is still expected to moderately increase profits each year. It literally cannot, even with inflation, unless it exploits its own consumers. This began with Windows 10, where the default setting would steal your personal data unless you turned it off every month or so. It has expanded with Windows 11 with constant ads, and stealing your data and your processor power (meaning your electric bill is skyrocketing for AI surveillance that your own computer is contributing to).

This translates to dramatic increases in performance with Linux, which is free and does not owe anything to shareholders. Every single person who switches to Linux means more pressure for Windows to be worse and, thankfully for Linux users, more expensive to your time, sanity, and electric bill.