Boxing and Muay Thai training both a requirement for MMA?

Many websites and articles describe the style used in mixed martial arts competition as a mixture of Brazilian Jiu Jitsu, Wrestling, Boxing and Muay Thai so as to cover the different aspects of kicking, punching, clinching, takedowns and ground fighting. Why are 'boxing' and 'Muay Thai' listed as separate entities that are requirements for MMA although just one would be sufficient to cover one's striking skills?