A single language would never work anyway. It would inevitably split into dialects and regional/generational/racial slang, which would soon become incomprehensible to outsiders.

English patois in some parts of the world makes no sense to British/American speakers, even though it reads and sounds more or less like standard English.

You don't even have to travel to find the split happening. Just try listening to teenagers.

Language isn't just a utility. One of its functions is to define social register and tribal identity, and humans are so attached to both they can't accept homogenisation.