40% of foreign students in the US have no close American friends on campus
It seems obvious that a huge (and hard to solve) reason for this is simply language!  There are many levels of language proficiency:   Level 1: Can read and write it at your own pace
   Level 2: Can comfortably converse in a professional setting (little slang or cultural knowledge needed)
   Level 3: Can comfortably converse in a social setting (slang, faster speech, less clarification)
   Level 4: Can do all of the above passively, being able to pick up valuable information just by overhearing conversation without focused metal effort
   Level 5: Ability to do all of the above in a noisy and hectic situation (like a party, sporting event, etc)

It's really, really hard to make good friends without getting to Level 4 or 5.  I work with many people for whom English is a second language.  At work they don't miss a beat and are great teammates.  But it can fall apart in a social setting.  Once there are 5 overlapping streams of conversation stuffed full of cultural references these coworkers of mine very frequently lose track of the conversation and become quieter and quieter over time.I bet you'd find that this 40% number is much different for students studying abroad in a country where their native language is common.  My US friends who moved to the UK or Australia had no problem making friends.