One guild plus a public trading system open to all is the established norm for MMOs, and there's a couple of good reasons for that.

First, a public trading system open to all means the economy works, and everyone can participate in it. It doesn't have to be an auction house, but it does need to be open to all.

Second, belonging to a single guild means you have a loyalty and attachment to that guild which in turn contributes to the overall sense of community in the game together with the longevity that brings to people playing the game. It also means that a guild can recruit individuals its officers know, or get to know, and in whom they can learn to place trust, and it can avoid the guild being robbed blind by people who have no sense of commitment to it whatsoever because they're in four other guilds as well and only joined this one for what they can take from it in terms of benefit or loot.

For the past 3 years I have had the opportunity to join up to five guilds in ESO, up to ten for most of that time if you include my second account. I can't think of a game in which I've ever felt less inclined to join even a single guild. Other than the few guilds catering for mainly real-life friends, or established guilds from other MMOs, the guilds in ESO are largely meaningless other than as a means to an end trading-wise.