The holy Isbitzer Ruv on this Parsha n notices that in pasuk four in the list of clothes, the pants are ommitted from the list. The Ishbitz says that they were not considered “garments” of the Kohein Gadol. According to Ishbitz, they were to only cover oneself and symbolic of lustful ideas. This really doesent answer the question why they are not included. If they represent an idea, they are integral.
The Shikkerdovid maybe would like to suggest that the pants are “foundational.” Meaning, to grow in spirituality there needs to be a baseline, framework or foundation. One cannot grow in spirituality or self awareness without basic principles. To the Shikkerdovid, the pants dont just represent a guard against lust, because lust is a natural, organic desire. It is a baseline of discpline, of “regilus” or regularity. It represents the daily requirements, prayer, learning, charity, chesed, which accumulate slowly and allow us to grow.
Simply put, you can’t go to shul, to chesed, to work without your pants. It is the first step in the daily process of spirituality.