What is reason for not having purpose as part of Daily Scrum event in Scrum Guide?

I don't understand why you say that the purpose of the Daily Scrum is not explicitly mentioned in the Scrum Guide. There are quite a few statements that appear, to me, to describe the purpose of this event:

At it, the Development Team plans work for the next 24 hours. This optimizes team collaboration and performance by inspecting the work since the last Daily Scrum and forecasting upcoming Sprint work.

The purpose is to inspect the work done since the last time, plan and/or forecast upcoming work, and optimize collaboration and performance.

The Development Team uses the Daily Scrum to inspect progress toward the Sprint Goal and to inspect how progress is trending toward completing the work in the Sprint Backlog.

Inspection is also highlighted here. The focus here in inspection of progress, toward both the Sprint Goal and the completion of the Sprint Backlog.

Daily Scrums improve communications, eliminate other meetings, identify impediments to development for removal, highlight and promote quick decision-making, and improve the Development Team’s level of knowledge. This is a key inspect and adapt meeting.

Having a regular, consistent, but short Daily Scrum eliminates other overhead for the team by reducing or even eliminating the need for other meetings that teams may find the need for and improving communication. Ensuring that impediments are highlighted no less frequently than daily helps the team spot and fix problems and optimize the chances of the team reaching the Sprint Goal.

I don't think it's necessary that every role and event have an outright "the purpose of X is Y" statement in the Scrum Guide. Sometimes, you can determine the purpose of the event by reading the full description. The Scrum Guide as a whole isn't that long, and neither are the descriptions of any of the particular roles or events.