That logic suggests Microsoft should simply never sell development tools, or even productivity software, because they would presumably be able to leverage the advantage of those tools internally in their own interests to crush all competitors.
Microsoft has long taken the position that its interests are served by there just being more computers being used by more people to do more things (‘a computer on every desk’ etc) - not by monopolizing all computer activity. That’s more IBM’s style.
Why I’d it better to ship it than keep it?
If you have a widget that makes software developers 10% more efficient, sure you can use it to make your own developers more efficient, but if your developers currently account for 1% of all the programming productivity on the planet you just realized a value equivalent to 0.1% of global programming potential.
On the other hand if you ship it as a product in exchange for, say, 10% of the increase in value produced, you contributed a value increase to the world equivalent to 9.9% of programming output, and captured .99% of the value of the market for yourself. That’s much more than the value you could have captured yourself and you grew the market meaning you can extract even more value on the next go around.