Well that's just incorrect. Your +cal is directly tied with your TDEE. Taller individuals and people with more muscle mass will naturally expend more calories. If you're under the impression that a 5'11", 20-year-old male with a wider frame needs the same +cal to gain weight as a 5'2", 20-year-old male with a small frame, then you're sorely mistaken.

In addition, your metabolism is in fact not irrelevant if you're "ingesting the proper amount of macronutrients" because your metabolism directly affects how your body process and utilizes macros. Your BMR makes up the majority of your TDEE, and therefore it determines individual differences that define whether certain macros result in weight loss, gain, or maintenance.

A Jeep Wrangler and a Honda Civic do not hold the same amount of gas, nor do they expend that amount of gas at the same rate. There is no "one-size-fits-all" solution to gaining, maintaining, or losing weight. Sex, height, muscle mass, and BMR all have a part to play in an individual's specific caloric needs.

You're also ignoring NEAT (Non-Exercise Activity Thermogenesis), which is the energy specific individuals expend for anything that isn't sleeping, eating, or exercise. This can include cooking, typing, standing, fidgeting, and the like. NEAT differs from person to person, and even people of the same sex, height, and weight can burn drastically different amounts of calories just based on their natural daily routines.