It's pretty easy to highlight passages with most e-reader software. Some even let you write a note that goes with the highlight. You can then look through a list of all highlighted passages.
It's when something comes up and you remember that you read something relevant in the past. In most of those cases it's not about "Ah, this is something I may want to check again at a later time, so let's highlight it". So, there's no list of highlighted passages to search through.
Indeed this is one of those things I miss about paper books. When the above happens I have a terrible time finding that on my Kindle. Most of the time I'm not able to, while with a paper book I didn't have that problem, mostly.
