It's more that "arrive" is intransitive, whereas "reach" is, in the sense you want, transitive (though in other senses it can be intransitive). Because "arrive" is intransitive, you can say "I arrived", period, but not "I arrived Seoul". To communicate the latter, you need a preposition, such as "I arrived in/at Seoul".
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TeacherScottinBusanWhy can I say I arrived IN but not reached IN?In the usage we're discussing here, reach already has the meaning arrive in.