Languages develop. During the middle ages, when the European languages developed into a relatively stable state, most people had a small area of influence with little contact to outside of this area. Consequently, the languages German and English developed independantly grom one another. (Some details below.)
If you take the words
bekommen: Both derive from the same Germanic root
biquomen, which was a prefix-root-combination itself meaning
to come about/around. In English the meaning developed into
become probably, because one had to come around in order to learn a profession. In German the meaning developed into
receive probably, because one had to come around in order to get special objects. (Personal guessing)
Details on the development: English and modern German developed out of Low German, which was spoken by the people around the cost of Germany and the Netherlands. English developed, when these people got to England and came in contact with Norman French and the Viking language. Modern German developed, when these people moved further south (away from the coast line) and came in contact with other Germanic languages. So, when Old English and Old High German started developing into Modern English and Modern German, both languages had nearly no contact at all.