In "The Silence of the Lambs", "Hannibal", and "Red Dragon" almost every character that I remember addresses Hannibal Lecter as "Dr. Lecter", even after he was imprisoned for being a serial killer. Wasn't his medical license revoked? Did they keep calling him "Dr." as a sign of respect? I find it a bit odd that this aspect wasn't taken into consideration...
It's a matter of respect
Even if his medical licence to practice has been revoked he is still a qualified doctor, having earned a medical degree, and so is entitled to be called "Doctor".
However, it's also a matter of maintaining equilibrium with him on a daily basis. Respect and proper manners are highly important to Lecter and treating him with disrespect (or indeed to people he likes) or bad manners is likely to make him very difficult to deal with.
When Clarice is disrespected by Miggs in the prison Lector tells her:
Discourtesy is unspeakably ugly to me.
and essentially talks Miggs into suicide.
Because at some point in his life, he got a Doctorate.
A Doctorate is a very high level of college degree.
There are doctorates in almost any collegiate subject. Physics, engineering, data science, geology, psychology, music, etc. In some cultures a doctorate is required to practice medicine at full privilege (unlike, say, a nurse practitioner in the USA). Whether the honorific applies to non-doctorate medical doctors, may vary.
Back when manners existed, people were formally addressed as "Mister", "Missus", "Mizz" or "Miss". However, when someone has earned a Doctorate, this is replaced by "Doctor". Using "Mister" for a doctor is simply wrong.
All they're saying is "Mister Lecter" but using correct English. Simple as that.
But he's a bad man. Shouldn't we take that away?
No. Using "Mister" is an insult (not least, against English itself). It means a) you don't know, b) you are developmentally impaired, or c) you are intentionally insulting or taunting the doctor in some way. For a good example of the latter, see how the Ancient One speaks to Doctor Strange during his training.
As for taking the honorific away from bad men, here's what King George V (World War II era) had to say on Britain's highest honorific:
The King feels so strongly that, no matter the crime committed by anyone on whom the Victoria Cross has been conferred, the decoration should not be forfeited. Even were a [holder of the] VC to be sentenced to be hanged for murder, he should be allowed to wear his Victoria Cross on the scaffold.
It's like that.
Doctorates, once properly earned, are for life and cannot be revoked, because it represents the work that definitely was done. A license to practice medicine is a separate thing, and can be revoked.
While I agree with other answers presented, I decided to turn my comment into an answer and go a little deeper into Thomas Harris' objectives on the FBI having incentive to treat Hannibal with respect, but also how that's a theme leading to a more provocative question about humanity and the notion of "civilization" or "civility"...
The first two books released were Red Dragon & The Silence of the Lambs. Both these works are mirrored works executed with an episodic structure (a specific case of book, if you will) and a larger arc of Hannibal getting caught (Red Dragon; at the beginning of the novel) and Hannibal escaping (The Silence of the Lambs; at the end of the novel) with Hannibal being asked to consult on both individual cases with two different FBI lead protagonists (Will Graham, Clarice Starling) while incarcerated.
These books have then been adapted into films with The Silence of the Lambs having one film and some of its elements have been placed in the Hannibal TV series, and Red Dragon having two films adaptations and a lot of elements in the TV series.
The reason that is important to answer this question is because
- All versions/adaptations provide PROOF of incentive for the FBI to treat Hannibal with the respect he believes he deserves so that he will help them catch other serial killers (Francis Dolorhyde, James Gumb) in these two cases. Both killers are caught, but Will Graham is initially set up by Hannibal and nearly dies in the process...
- Red Dragon (book and film adaptions) points out that there was an unknown pre-history between The FBI/Will Graham/Jack Crawford and Hannibal, implying that Hannibal was helping them with cases, before he was caught by Will Graham. This is also the basis for the starting point of Bryan Fuller's Hannibal TV series, a prequel to Red Dragon with Hannibal working with and around the FBI to help them solve cases (and to set them up against other serial killers, as Hannibal is experimenting). This helped earned him additional respect and form attachments and proves that he is a resource the FBI needs.
- Thematic. One of Harris' greatest themes with this character is the brutality of Hannibal's actions juxtaposed with a well-kept, jack of many trades (surgeon, psychologist, artist, chef, art collector, philosopher, ala scholarly disposition), verbally-civilized, brilliantly intelligent character. So Hannibal's infinity for not tolerating rudeness, and therefore promoting "respect", and being able to spread that ideal to those that come to exist around him, is a part of showcasing this theme. Hannibal is a bit of a Mephistopheles or Faustian character Waring against the limitations of the universe of "God". Harris may be using all of this to ask, if humans can be spiritually elevated by art and civility or is that a false facade for a more complicated game of survival of fittest, where we are still fighting for power and control among ourselves? (IMO Harris answers it at the end of the Hannibal novel, but he might regret it since Cari Mora (his latest novel) kind of reads like a Clarice/Hannibal revisionist work).