In the past few decades, there's been a broad movement to stop using unnecessarily gender-specific language in male-dominated fields. "Guys" is an interesting one because many people really do use it in a gender neutral sense. But for a significant number of speakers, the word does have a clear gender restriction. To these people, greeting a room with "Hey guys" sounds about the same as "Hey boys". You can imagine how this greeting would grate a little if you were not a man.

It is not really about people getting offended. The only people who really seem to get offended are the people who insist on using "guys" in spite of a polite request not to.

Language diversity tends to get overlooked quite easily. Remember that just because "guys" is gender neutral in your dialect doesn't necessarily mean that it has this feature for all English speakers across the world. You can't assume as much shared linguistic and cultural background in a programming language Slack channel as you can when you greet your friends at a bar.