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.