A simple answer?

BECAUSE IT’S HIGHLY PROBABLE THERE ARE IMPORTANT CHARACTERISTICS OF YOUR COMMUNICATION STYLE (and PATTERNS) you may be unaware of. This involves, among others: body language,  content, structure, voice, presentation style, personality, charisma, rhythm, flow, emphasis, passion, energy, and most importantly perhaps, your ability to understand the correlations between all of the above. Last but not least, the best speakers in the world have at least one thing in common: they acknowledge (and re-acknowledge), again and again, the importance of continuous learning and humility.

So what's your real challenge?

To find a public speaking coach whose ambition and skills go beyong putting you in an artificial corset of ready gestures, waving your arms like this, holding your hands like that. The ultimate goal of a reliable public speaking programme should NOT be to impose anything on you. Inevitably, you will lose the very last traces of what you used to refer to as authenticity and personal characteristics.

Instead, it should significantly improve your observation skills, in particular self-observation.