First, I think that it hinders people to think of one language as a "native language." Languages become native to speakers through practice. Calling on language "native" promotes the idea that it is more natural for someone to speak or learn one language than another. This also promotes the association of language with ethnic identity, when those are actually different things. A person can have a certain ethnic identity but develop a different "native language" because of the language spoken with them at home, school, etc. Langauge is really just a means of communication, nothing more. Ethnic identity may be important, but it doesn't really need to be tied to language use or proficiency.
Because they can. People waste energy consuming a language they are already proficient in instead of devoting that energy to becoming familiar with and practicing a new language. If people would do this from an early age, they would speak numerous language by the time of adulthood. It would be possible if children could change schools after gaining sufficient proficiency in the language of the school, or if schools would designate different languages to different age groups. That way, they would practice learning and interacting in one language for a few years, and then switch for the next few, etc.This is a chicken-egg problem. Migration is restricted out of concern for language preservation and ethnic-territorialization of economic opportunities. If economic opportunities were not threatened by migration, more people would consider it feasible and national protectionism would diminish (hopefully). If migration restriction was still politically popular, it could at least be facilitated between cities/areas with widespread proficiency in a given language. That would mean people in Amsterdam could migrate to Berlin if they speak German, but also Helsinki, if there was widespread German proficiency in that area. Likewise people living in Helsinki who wanted to learn Dutch could live in Berlin if Dutch was widely spoken there. Each language would have its own global topography, and the topographies of different languages would overlap since each city/area would have multiple language proficiencies.You could move to a Greek island and practice Greek language, but you could also learn and/or practice Korean there with other Korean speakers. If your Korean became sufficient, you could move to a city/area where Korean was spoken along with other languages that you had no familiarity with (yet). Say you moved to Beijing and spoke Korean at work and among a sub-society of Korean speakers, you might then work on learning/practicing Chinese or Swedish, if there were sufficient speaking-opportunities.
The idea is that all areas/cities would be multi-lingual without everyone having to speak all languages. There should also be measures to ensure that minority-language speakers in an area do not become institutionally isolated from interaction, as that would promote language loss and domination of some languages over others.