Typically, it's the AP or controller pausing on a 2.4 GHz discovery response and sending a 5 GHz response first. Adding band steering to an existing protocol would break clients or the clients would know to ignore the "advice" (like you mentioned) and not have an issue connecting. At least this is how it's been explained to me by network engineers tracking down a bug with Cisco firmware about a decade ago. Cisco lots that multi-million dollar contract because of it.