I mean it almost seems forced, affectless and emotionless robotic droning. 
You notice it in language dubs done cheaply, but even people off the street don’t talk like that.
I’m not asking for stellar voice talent but even people reading a book out loud don’t sound like that, it is a very forced specific sounding robotic tone.
The latest film I noticed this in was the english dub of Fantastic Planet, it was kind of embarassing trying to convince someone to watch it.
It depends on the budget for voice talent. For older films, dubbing was done as cheaply as possible, so the people involved were not top talent and the result showed it.
Various reasons:
Underpaid voice actors. An obvious one.
Lines recorded in isolation. To save time/cost, it’s common to record all of one person’s lines in one go, sometimes not even in chronological order. So they don’t have any context to work worth, can’t react to other actors, etc.
Trying to match a translated line to the lip movements. In the worst cases, you get something like the videogame Final Fantasy X, where the english version’s line was something like “I agree with you” and the japanese version was “hai.” So the english actress had to squeeze a whole sentence into the time it takes the on-screen character to move her mouth once.
As I understand it, voice actors in the US are paid relatively poorly, so it’s not a career that many people aspire to. Most actors prefer to do live-action work, because it is paid much better. The ones that aren’t talented enough to do the live-action roles are left to do voice acting. (That’s not to say they’re all talentless, of course! However, there is only a small pool of quality voice actors, like Tress Macneille, Dan Castellaneta and Jim Cummings, and they are in very high demand.)
And within voice acting, dubbing over a foreign film or TV show pays less than acting in a US-made animation. Most dubs are done as cheap as possible, because there’s not much of a market for them (with the exception of big-name stuff, such as Disney’s dubs of the Studio Ghibli films).
So basically, dubbing foreign films and TV into English pays peanuts, so only the crappiest actors take that sort of work.
It’s a different story countries like Germany, where the actors who dub over Hollywood films are household names, and presumably much-better paid.
Back in the day, I interviewed a woman who dubbed Joan Collins’ voice in Dynasty - she had a lucrative career, it was a hit show in Germany and they spared no expense on translating and dubbing. This woman was earning big bucks and yet, no big personal fame from the show.
Translation is the first big hurdle - there are many ways to translate dialogue, and often it is a matter of finding the right idiom, but more importantly, trying to match as closely as possible the number of words. No easy feat. Sometimes you have to pad a word or two there for some a phrase you can say quicker, or sometimes you have to really search for a way to say something in fewer words that would still make sense.
So often it is not the actor’s fault if the dialogue is stilted or doesn’t fit the mouth on the screen. Hard to try to fit in 38 words when the actor on the screen is only speaking 18 words. Same problem when trying to make 13 words in German match 42 words in English. They would do each scene in about 2-3 minute loops. Sometimes would take many, many tries to get a particular scene to line up correctly. The actor had to watch the lips closely, practice saying the lines in their language and trying to match it up. Far trickier than it might seem at first glance and takes time (and money) to get it right.
Funny you should say that, OP - I agree that a lot of voice dubbing is terrible (e.g. any anime ever), but I liked the English in Fantastic Planet. Its 60’s radio-anchor style weirdness was kind of fitting to the surrealism, I thought.