What are the main reasons people do an LLM in the US? It seems very very expensive and I'm starting to wonder if it is worse it. It seems hard to find data on the outcomes of people that have already achieved the LLM

Hello CountryCat, There are quite a few reasons to pursue an LLM. There are academic issues such as to build kowledge and focus on a specific area of law that you will highly benefit from in your later career. You can improve your grades if your degree wasn't that good. You will be able to gain knowldege of other legal systems and it's a prerequisite if you wish to gain a Phd. An LL.M. is very common if you want to become a solicitor or barrister. It can also help transition into a new practice area.
The career factor is even more important. An LLM on your CV looks good. Even though it might not be a criteria for selection, it highly increases your employment chances. If your LL.M. is from a recognised University you will stand out even more. This helps to increase your salary, too. For some people it may be a stepping stone to a caeer change. This is more common for professionals in areas like education or engineering.
As a foreigner it is a great opportunity to travel to a foreign country and gain knowledge of their legal system as well as improve their English. It's a life eperience.
One of the main reasons why students pursue an LLM in the States is the reputation and ranking of many US Universities. I suppose the LLM has been established in the states a lot longer than in other countries. Some students decide to sit the US. bar exam for which postgraduate studies are a prerequisite. Last and not least you may conduct research and get it published. I am sure there are many more reasons to pursue an LL.M.