This is a bit indirect, but I've had some luck talking with stem people from the perspective that the purpose of stem is to actually use it, improve lives, get something done in practice in a material way. Given this, the technical portion of getting something done is at most 20-30% of the effort required in total, with the remaining portion coming down to various aspects of sharing knowledge, psychology, teamwork, etc. In short, society and the social sciences. If someone wants to play in the stem busybox and never get anything actually done, fair enough. If you want to actually be effective and actually implement or use some aspect of technology, you need to understand the context in which it will be used.

In short, tell stem-brained people that if they don't learn anything about humanities that they are deeply unserious.