Why use 碰头 (pèngtóu) to mean "meet" instead of say 见, 见面, or 看见? (In e.g. 过两分钟我们在外面碰头)
Actually, the 碰 itself can mean to meet. E.g. 我们周六再碰一下吧。// let's meet again on Saturday. 

It can form the words: 碰头, 碰见, 碰面. The dictionary definition of 碰头 is 会面；短时间地聚会(short meeting). The question 'why' in any language is hard to answer and sometimes even unanswerable. For this case, we can only imagine two people run into each other and figuratively it implies they 'meet'. Usually, 碰 happens in brief time(they bump into each other and then split immediately), which reflects that the meeting is short. That could somehow explain why 碰头 can mean "short meetings". 

My two cents.