I think it's just because it doesn't sound very natural in English. You would say 'you speak a lot'.
It is decidedly relevant when translating into English. To translate "La viro rapidas" into English as "The man quicks" would and should be marked incorrect. I'm sure there are situations where a too-literal translation into Esperanto from English would also be incorrect.
Just because "The man quicks" is a literal translation from Esperanto does not make it an acceptable translation. Similarly, "You speak much" is not a translation into correct English. The point is to translate into correct English/Esperanto.
It's the same with any language which works differently from English. Russian does not use articles and doesn't usually use the verb 'to be' in the present tense. That doesn't mean that "Мальчик голоден, он ест яблоко" should be translated "Boy hungry, he eats apple", because that isn't correct English.
Oh, right. I had gotten your reasoning backwards.For some reason I was thinking you were saying that it couldn't be like that in Esperanto because it didn't make sense in English. I don't think, however, that it would sound unnatural in English. That might be because I'm not a native speaker, though.
In American English (and perhaps in other English dialects) "to speak" and "to talk" have different meanings: speaking is typically one-sided (a related idea is "to give a speech"), whereas talking implies that one is conversing (at least expecting to converse, or believing they are, even if they talk to much).
Does Esperanto make such a distinction?
For what it's worth, I am a native English speaker, and I think "much" is completely acceptable here. In fact, it is the word I would use in real conversation.
I admit it is not colloquial, but it is correct. I often get strange looks for speaking this way because it sounds unnatural.
The distinction to make here is that "a lot" is colloquial.
I dislike the use of "a lot" because it references a physical quantity or a collection of something. One cannot quantify speaking. One can quantify words or time, but even those are not physical objects that could be sorted into lots.
By contrast, one can have "a lot" of objects. In which case it is appropriate to use the phrase.
Example: "I eat a lot of oranges, but I do not eat much."