From the Wikipedia page on USB: Additional USB hubs may be included in the tiers, allowing branching into a tree structure with up to five tier levels. So by

From the Wikipedia page on USB: Additional USB hubs may be included in the tiers, allowing branching into a tree structure with up to five tier levels. So by "infinite" they mean "five", unless the connectors can make use of a non-standard communications mechanism between each other which makes them appear as only one big hub. The other downside of daisy-chaining is that you have to unplug everything to remove a lower-level device.