They can use this information to their advantage.
Salespeople aren’t your friends. As much as they try to seem like they’re on your side, their goal is to get you to buy, even if you might not want to.
By telling them you’re a lawyer, doctor, engineer, you’re giving them additional information they can use when trying to see how much they can get you to pay.
The less you tell them the better. They don’t need to know your job.
It’s like telling NYC cab drivers you’re from out of town. They take you the long way to up=charge you, instead of taking you the cheaper, more direct route. (Luckily there’s now a feature in Google Maps and other apps available that tell you if cab drivers are taking you scenic route)