My own experience. I have seen exam scores in my classes put on the blackboard. I have never seen 100% on from anyone from any of my exams (except for a rare trivial exam where there are many 100%s). As a TA, I have occasionally missed getting 100% in exams professors have given to students at the freshman level. I have had professors tell me they took the same test I took when I was a college junior and they told me they did not score 100%. Sometimes exams are made too long, a variety of reasons preclude universal 100%'s.
If you are getting 100% it means the exam has been made too easy - so it is not a good test - or that you are working in a class that is below your abilities and you should be in the next level up.
However - I have seen cases where someone consistently scores 100% in finals but you can see their test results through the year climb.
It's very unusual and I have always suspected that these students were just not getting challenged by the material.
Sometimes there is scaling so the highest scored student automatically gets 100% and all the other students are adjusted to fit some preconceived idea about what the mark distribution should be.
It's done the other way too - to make it harder to score top marks. I've seen an exponential scaling for practical work - so it was fairly easy to get 30-40%, but getting that past 90% was really difficult - requiring things like "originality" and "flair". I once gave 100% to a student who redesigned the experiment in such a way he demonstrated that he understood scientific method and the general principles in the theory. His results were way worse than everyone else but I figured that didn't matter.
Lots of people complain about the time - I used to set exams so that it would take me less than 1/4 the available time to complete them. i.e. it take 1/4 of the set time to write the expected working to the answers when you know what the answer is already. The other 3/4 time is for the student to figure it out - and there were always built in shortcuts for alert students.
I had a prof for postgrad QM who gave assignments his students would use 5-10 pages to complete - and the model answers would come back on a single sheet.
I, personally, once scored 125% in a final exam - it was a practical and it had bonus marks and penalty marks.
125 was the maximum score - so you could call that "100%" if you like - but I still did not pick up all the marks available for each section. The highest percentage of available marks I ever got in an assessment was 98% - that was a restaurant manager test, quite easy questions but the pass mark was 80%.
The near impossibility of getting an even 100% is why scores get grouped A+, A, A-, B+, etc.
I knew someone who managed to collect every single letter-grade that the University awarded - including every possible way to fail. He treated exams like sport fishing - he'd deliberately target a particular grade.
One way I learned to get higher grades was to answer the essay question.
We used to get exams like "answer any 5 questions out of the 7" and one of the questions was always
"write an essay about some aspect of the coursework that you enjoyed (20)" ... nobody would do it. It turned out that because nobody did it, it was marked so very leniently that almost anything would get full marks.