Localmonero/Agoradesk trader here.
Every payment method and country have its own market price. For an example central exchanges doesn't accept Paypal, Cashapp, gift cards, Cash at ATM etc. Most of them only accepts bank transfers. So you're comparing apples to oranges.
Even if you compare bank transfer offers you have to keep in your mind; some banks blacklist central exchange's bank details, so you can't make money transfer and/or it has much less strict KYC which is more privacy friendly.
Also you can benefit from OTC advantages like there's someone can help you directly and/or can offer custom solutions for you.