I fully agree.
In addition, this is an example of "be careful about your wishes, the might become true": In my guild, people used to be quite talkative and every single achievement, no matter how unimportant, would trigger up to a dozen "gz" followed by a "thx" and sometimes even some "you're welcome"s. On a busy night, you could easily spend your whole gametime just typing gz, hi, wb and bye. Fortunately people got tired of this after a while and today you will only receive a gz when your achievement deserves it and only 1-2 people will reply to your "hi" which we think is a fairly comfortable level.

I think the bigger your guild gets, the more likely the players prefer to remain silent because they think "if everybody psts a reply now, the gc will become quite crowded so I better remain silent." So I'd say in large guilds is up to the officers to take care of the courtesy chat even more than in smaller guilds with less hierarchy.

Last but not least, I someone complained to me about such a seemingly silly thing, my reply would be "Don't tell me you don't get enough response, instead tell me the ratio of gz you're giving to others against the gz you're receiving. If your ratio is better than 2:1then quit your whining and look for bigger issues to resolve."

Not speaking in guild chat is an interesting situation in that it sneaks up on you. I feel that it's very rare for a guild member to be doing it deliberately. Guilds are interesting- in "social" guilds especially. In such a guild, if you don't frequently dungeon or raid with the other members, all you may ever see of your guildmates may be scrolling green text- and as I noted below, that will sometimes be produced by addons. In this environment, it's easy to forget to return courtesy. Challenge your guilds with interesting guild chat! Make them want to participate! It's the only way to make this work- but making sure you do your part comes first.