Because Microsoft paid a lot of people to develop and maintain a software they gave away for free to users?
If Microsoft let me buy Windows without that, I would have paid for it. Alas, If I spend $200 on Windows 10, I get the same tracking landmines than the person who spent 0 a few years ago.
That they have given an ad ridden OS away for free is their problem, not something they were forced to.
A free version with ads and telemetry isn't free, it's just monetized differently. And the vast majority of the cost of developing it is either sunk or defrayed by the paid versions.
The default exists because not everyone has the same reaction to the telemetry as you. For those who do, they have some means of bypassing that (for now).