Creating a party with people you don't know is called a PUG. PUG stands for pick-up group. For a couple of reasons, many people don't like forming pugs. One of these reasons is that people feel that they can't trust people in pugs, who might not know how to run an instance or try and loot everything and leave nothing for anybody else. Secondly, it is pretty easy to form a very good party with guildies, who you know and trust. Therefore, there's really no need to look for random people to form a party with. Thirdly, what people above said about gold spammers.
If you're struggling to find a party to run an instance or complete a quest, join a guild and I'm sure you will find people who will run instances or help you with a quest.