Hopefully it's not too much trouble to make this optional per group because it really isn't something that would work for our group culture.
Many reasons... but here you go, some examples:
I've seen this option to ignore a specific user available in a forum that doesn't have a mailing list feature. As a moderator of groups, I was really curious about it as I researched technologies available. What I saw happen on that forum is that it created a lot of confusion in the posts and replies.
It's not that easy to follow a threaded discussion if those participating are not seeing many parts of it - yet they are participating in it. If you are participating in a thread, yet not seeing all the contributions to it because you've chosen to ignore certain posters, your replies may cause further confusion, redundancy, etc. If you can't "see/hear" what someone else just wrote because you are ignoring them, then you have no idea if what you are posting is the exact same info someone else just did. If your opinion is the exact opposite of what the person you are ignoring just posted, but you have no idea because you are ignoring them (but they don't know you are), then perhaps your reply will be less community minded. I've seen all this = added work for the moderators who then had to deal with duplicate replies, replies that don't take in to account what the prior person just contributed to the discussion ( not everyone quotes), replies that were unknowingly insensitive to what the prior poster had written, and so they then also as mods, have to deal with a somewhat dysfunctional conversation/environment, and make sure no one is using the feature in way that is hurtful - which leads me to my next point.
There is an aspect to this feature that can be felt as hostile or rude: say the person you are ignoring asks a follow up question on a reply you have posted - you don't see that question - so you don't reply. It becomes obvious to everyone who is reading the thread, who is ignoring who even before it's apparent to the person who is being ignored. On the above forum I was checking out, posters were even saying things like "perhaps I'm confused because i have xxx on ignore...". So that would add yet another thing our mods would have to watch out for to ensure no one is made to feel badly. Plus the fact that as mentioned the replies of the person who is doing the ignoring might inadvertently not be mindful of what the ignored person just wrote because she/he can't see them!
I think it's great that you can follow / mute a thread in groups.io. But muting or ignoring an individual can result in the online equivalent of alienating someone - embarrassing someone in front of the group - it has the potential to verge on bullying or being felt as such. Frankly, it just adds a bizarre level of dysfunction to conversations that would make our work as mods so much harder.
When a group's focus is on community building, and moderators are already overworked, perhaps you can imagine how all of the above would be incompatible with a group's core culture and values of never making anyone feel like their contribution is "less".