For me the main problem is that going out for lunch in person with coworkers (even ones from other departments I've never met) is completely fine and I like to do that (when I was in the office I did that 3-4 days per week) but when I am having lunch at home I don't like it at all. I can't put my finger on it but it always feels like useless smalltalk and not making any connection where in person that works quite well. And I'm neither pro-office or anti-WFH or even someone who doesn't make friends online... but going out for lunch or a beer or just an afternoon coffee break is something that I can't reproduce via video/audio.

You're still welcome to ping people on Slack during lunch and see how they like that. More often than not though, I think you'll find that they're going to respond to you once they're back on the clock. It's fine if you're a friendly person, but I think you underestimate how much social pressure is involved in talking to people during their lunch break. The only reason I really engage in conversation while eating lunch is because the alternative of "I'd rather just enjoy my food and save the work talk for work time" is a faux pas.

I'm pretty pro wfh, but for a lot of people (myself included), the value of a slack conversation is less than a in-person conversation, so as a recipient I'm less likely to make time for it. That doesn't reflect on the worth I put on the person, but rather the type of communication.