agreed, but let's assume you don't know what an AVL tree is or what the trade-offs are between an AVL, red-black or other type of trees: how long does in this age to find that information if you know what you are doing? I mean, you could allow your interviewee access to a computer and based on the type of googling they do it should be fairly obvious if they know what they are talking about.

In your day-to-day you have wikipedia, stackoverflow, hn and so on: software development hasn't qualitatively changed in the past decades to require multi-day interviews when 15 years ago a single 30-45 min interview was more than enough.

It would actually be interesting to compare the length of, say, a google interview a year after company inception compared to now. I am sure that despite the fact that nowadays the impact of a bad hire would be minuscule compared to back then, the interview process is way, way, way longer and more difficult.