ATC often has such thick accents you cant understand the radio
Indeed. Increasingly, mis-understood ATC/Pilot/ATC radio traffic, is becoming a causal factor in near misses etc.
As a side note, I work on the ground at an airport and have frequent contact with pilots on the ground to air radio, I can say that more often than not I have to request pilots to, "say agian" in order to understand their comms.
English phonetics vary too widely from dialect to dialect and it takes too much effort for too many people to achieve an acceptable pronunciation
As a rule, comms with ATC is very formatted, and is designed to communicate technical information about the aircraft's status. Because of this, range of vocabulary is limited and strictly speaking a pilot need only be competent in speaking english at this level. However, most airlines demand that their flight deck crews be fluent english speakers.