Enterprising student journalists at SUNY New Paltz have discovered that some full-time professors pretty regularly miss their office hours. 
UD can recall missing her office hours once or twice in her career, due to ... I don't know... cluelessness. UD zones out on occasion. She forgets things. But with office hours this has happened very few times. At New Paltz it seems to be a trend.
Professors' reactions to the study, as reported in the student paper, are stupid. They point out that students rarely show up. This has nothing to do with whether you're supposed to be there.
An English professor, in a depressingly garbled remark, says "I feel that [ English professors are supposed to discourage students from using empty formulations like I feel.] in the English department, we spend more time in our offices, people are here frequently. It's not my understanding that people don't keep their office hours." But, as the article goes on to note, the professor's understanding is mistaken... That's what they're trying to tell him... They'd like him not to share his understanding, but to respond to empirical results. And note his argument: Hey. A lot of us hang around. This must mean we're keeping our office hours.
When lots of professors at a particular university fail to show up for their office hours, chances are you've got a generally demoralized crew. (Under Communism, Poles used to say of their bosses: They pretend to pay us, and we pretend to work.) One of the glories of a tenured professor's life is that no one's really watching you; you're given remarkable independence and autonomy. If you don't much like your university, or your colleagues, or your students, you can, to varying degrees, withdraw... And UD would even argue that there are reasonably responsible ways to withdraw from some aspects of university life. Blowing off office hours is not one of them.