Very true. But, non-the-less, Jersey Fresh is correct in saying that the USEF rules SPECIFICALLY say that you do not need to wear a jacket or neckwear from BN through Prelim.
Subchaper 9 Dress
Dressage
5b Beginner Novice through Preliminary - Any single-color jacket is permitted. Tweeds, pinstripes, tasteful, and discreet accents such as such as a collar, cuff, lapel, piping, of any color, or crystal decorations are acceptable. Tailcoats are not permitted. Athletes may compete without a jacket. In such cases, a long or short sleeved shirt with collar and without neckwear, any color and design; tasteful and discreet accents, such as a collar, cuff, piping, is acceptable, neatly tucked into riding breeches is acceptable.
Show Jumping
7b. Jackets. Any single-color jacket is permitted. Tweeds, pinstripes, tasteful, and discreet accents such as such as a collar, cuff, lapel, piping, of any color, or crystal decorations are acceptable. For Tests, Beginner Novice through Modified, at any Horse Trial format: Athletes may compete without a jacket. In such cases, a shirt – long or short sleeved with a collar and without neckwear, of any color and design; tasteful and discreet accents, such as a collar, cuff, piping, is acceptable, neatly tucked into riding breeches is acceptable.
Whether “everyone else” will be wearing a jacket and neckwear at the AEC is anther question. But neither the TD not the judge can penalize you for not wearing a jacket and neckwear.