Why are the security briefings viewed as such a big deal for Trump and almost never mentioned when Obama would skip going to them.

Like everything else that is nothing more than ritualistic in Washington, the daily security briefings in a mostly normal world are nothing but MOS (more of the same).

Like it or not, like him or not, the security briefings being a daily ritual and not much value adding during the course of normal daily lives....a take action, aggressive, no nonsense, executive style will pick and choose where to spend their time. Are they need? In some form, no doubt. Daily? Most of the time no. Or a memo or a text or however they top brass in Washington communicate the usual day to day stuff.

It might be an interesting exercise to find out how many people and how many hours and how much money spent to prepare, package and present the daily security briefing.
I would bet it is a stagering number......hence, self perpetuating, establishment, business as usual.

Real executives do not put up with for very long non value adding activities/persons.