Volunteering is fine!

People use their bodies for work, true. The difference when it comes to surrogacy is that

1. each birth costs between 2 and 4 years of life (let me know if you want links to that research),
2. giving birth is the most intense endocrinological process in the animal kingdom. It can be psychologically devastating, especially if you need to give up your baby.
3. there’s no good reason to do it. When you hire someone to work with their body (for instance, construction) we consider the risks associated with that work and weigh them against the need for that work to get done (its utility). Is there an alternative? Is it really worth it? How much would people get hurt? What’s the possibility for exploitation? Simply put, whereas many types of physical labor have to get done and aren’t all that harmful, we tend to make illegal the ones that don’t need to get done or which are harmful. For instance, we don’t look kindly on mining for radioactive material without protective equipment. We have OSHA, we don’t allow you to be paid to chop off your own body parts for someone else’s amusement, etc.
4. it’s dangerous! In the United States donating a kidney — yes, having a kidney cut out of your body — is safer in the long (and short) term than giving birth. If we allowed surrogacy, we would have to allow organ selling, which is technically safer. That’s the death knell of civilized society. Political arguments will be, “why should we help the poor when they can sell their organs or do surrogacy.”

Regarding sex work, you’re right; it’s so profitable and ubiquitous that by making it illegal we just create a black market (like drugs). Since this black market is one of the main sources of harm for that industry, by legalizing and regulating it we also make sex work less exploitative and dangerous. In contrast, surrogacy is neither ubiquitous nor profitable (hence the exploitation of poor foreign girls).