We were allowed to make cheat sheets (technically cue card size) for one of my first ever chem exams in University...but the thing is when you are in a 50 minute exam and you have like 6 pages of written to do it is really not worth it to search around on your cheat sheet for something that you will probably get wrong. Basically if you have to search and search your sheet you do not know your material. Some exams are allowed open texts (one atmospheric science class I know of) because profs know if you want to waste time looking for answers in a very short exam you are likely going to fail anyway.
For the last science test I did that was open book, I basically tabbed everything and memorised only where things where, so I could flip straightaway and copy / read what I wrote haha. Aced that exam.