You are interviewing at companies that so many people in the field from around the world apply to. So it's really damn challenging to even get an interview lined up. It's up to you, often you can have more fun, and play a bigger role in product design, architecture, and planning at smaller companies. But if you care about the status, pay, and comforts of working at one of the big ones, then go for it. It really depends on what you value.

Also, based on my experience with interviewing and being interviewed, the questions are difficult on purpose. If everyone got the solution, you couldn't differentiate between applicants, but if they are very tough, you can get a sense of a persons problem solving skills, how determined they are, what types of questions they ask to proceed through the problem. etc. It shows a lot about a persons thought process and how likely they are to be successful when a tough problem is thrown at them. Often, getting to the solution isn't the point of the question. Seeing how someone solves something, they haven't encountered before is much more informative (especially for companies and startups working on things that are game changers).