This assumes that people who have a question they need answering will consider searching for an answer or getting to grips with the UI first, rather than heading straight to ask customer support. That's often not the case. Long before support chatbots became widespread, telephone trees made a point of telling every single user in the queue the help section of their website existed...

Agree bots should often make it easier to escalate and should default to escalation rather than blowing up[1], but if a large proportion of your queries are actually answered by telling people the FAQ page is a thing, it's a bit difficult to justify paying humans to do it.

[1]if escalation takes a couple of minutes for a response escalation and blowing up might be indistinguishable...