I suspect part of the reason they made cross-realm guilds was to deal with the issue of low population realms. I’m on the EU servers so don’t know how it is here in the US, but we have quite a few realms that are essentially dead or dying. Blizzard promised back before Shadowlands that all these realms would be connected up to higher population realms to improve their situation but they had increasing problems doing so until, eventually, they just stopped. There was no explanation - one minute they were regularly updating people with which realms were going to be connected next, etc, and the next the communication just stopped.
The intervening time since then didn’t make those dead realms any better and they had no solution. They did the semi-connection of the auction house, but that doesn’t help the situation for a lot of things, and certainly didn’t help guilds on those realms, or players who felt they didn’t want to/couldn’t move realms.
Cross-realm guilds helps both the people “stuck” on those realms, and the guilds who were often taking their last gasp.
So for Blizzard, this “solved” that issue of people/guilds on dead realms, without actually doing anything physical with the realms. Obviously it doesn’t solve it … but as far they’re concerned it probably dampens down some of the complaints.
At least that’s my assumption … but then, I’m also fairly sure they don’t do anything anymore “for the customer” so perhaps this is a pre-cursor to some other change that’s coming, that will save them money.