At 18 mos you wouldn't be letting your daughter out to just play in the neighborhood. She needs supervision. That is the reason for playdates. If you know the other parent you can just leave your child with them.
Once she's older she can just be out to play in the neighborhood if you know your neighbors. The whole idea is to keep our children safe.
Getting to know your neighbors, parents of your child's friends is not giving them the third degree. Do you hang out with strangers? I doubt it. You talk with each other to get to know each other. With a child, you're doing some of the getting to know the other family for your child. Your child, even when in grade school, is not mature enough to make judgments about the other family.
The world is different now than when you were a child. When my daughter, who is now 30, was a child, I knew my neighbors already when she came to live with me. I'd lived in the neighborhood for 15 or so years. My closest neighbor was retired, walked a lot and filled me in on neighbors that I didn't have direct contact with. Others I'd met while working in the yard. We had no new people in the 2-3 blocks surrounding my home.
My daughter got to know kids at school. Kids from the same neighborhood go to the same school. That is when we had playdates. The mothers would chat while their kids played the first time or two. After that the kids would spend time at each other's house.
Playdates do not have to be intimidating. Look at them as a way to casually know the other family so that you can be assured that your child is safe.
Not only physically safe but also emotionally safe. You want her to spend time with healthy families. You don't want her to witness fights. You want to know that the home is free of hazards, such as guns. And that the family is safety conscious. ie. they won't give your 5 yo a sharp knife to play with play dough. You want her to learn your family's values. The only way you can know that she's safe and learning your values is if you know the other family and their values.
Unless you're good friends with the mother of another toddler, it's not likely that you'd want to supervise two toddlers alone. Toddlers must have direct supervision. They're just learning how to play with others. Actually, they play side by side at this age with very little interaction. What interaction they have is clumsy and can be difficult for both children.
My grandchildren do play in the neighborhood. Because I don't know the neighbors as well as I did when their mother was a kid, I have more rules. The must ask before they go into someone's house. They must come home if there's a fight. I don't know the parents well enough to know how they'd handle fights, for example. That sort of thing.