SSH is one example of TOFU (Trust on first use) that does not require a CA (or indeed any third party) to be involved in establishing a trust relationship.
Most Pre-Shared Keys (e.g. as used for WPA in wifi, or used in a VPN solution) can provide some protection against MitM attacks if used correctly, and without needing to establish trust via a CA (or PKI), and without the need to involve cryptographic tokens.
Consider too that MitM can be facilitated by a CA (for example, when a CA is made to issue certificates without carrying out the required checks to validate identity - because of a compromise, legal pressure, or malice), and that using a PKI does not necessarily offer any protection against MitM - it simply establishes that a given token (a key) is reasonably believe to represent a claim (of identity), and poor crypto choices could, for example, nullify any and all of those claims/assertions.
As pointed out by @Steffen Ullrich in the comments below, TOFU is susceptible to MitM during the first connection. This could be mitigated (by for example providing some mechanism to validate the host keys being presented).
Following that first connection, you are either safe from MitM, or totally exposed.
I had meant to point to TOFU as a different approach to establishing trust (leaving users to figure it out, basically), but Steffen is absolutely correct that TOFU and PKI take a fundamentally different approach to validating identity initially, with TOFU making a design decision to not address the risk of MitM during the first connection - so the two trust models have a different impact on the ability to guard against MitM.
Since the question was about why much MitM litterature mentions PKI, this is a partial answer: of the more common approaches to protecting against MitM, a PKI-based solution provides more protection than other approaches like TOFU.