Because this kind of behavior was rampant in the past - where one company owned everything and could leverage it's size and influence to crush competition across distinct market spaces. It prevented other companies from operating in the same space, which led to stagnation, outrageous human exploitation, inequality, and ultimately the great depression.

It is now illegal as laws have been introduced with the aim to prevent this from happening again. The effectiveness of these laws, with regards to how well they fit the current era, is a different matter.