Great writers do more than just say, "in real life people don't do X, so my characters mustn't do X". They understand why people don't do X, thereby informing their understanding of their own characters' likely in-universe behaviours. (Of course, sometimes writers do something unrealistic anyway, but let's sideline that observation for the moment.)
Why aren't people eloquent in real life? Sure, it takes effort. But if you genuinely believed you needed eloquence in a given context, either to be understood or to suitably affect others, you would find a way. People do it; people watch every word when they're slowly working through a sentence in a foreign language to ask for directions, or when they're answering such an obviously slow question coming from someone who can't be expected to otherwise follow the reply, or when they're making a "conventional" political speech. 873
On the other hand, when in most real-life circumstances we can get away with something easier, that's what we do.
To be clear, "ineloquent" speech isn't a sign of being thick or uneducated, or talking to someone else who is - not, at least, if the feedback you've received meant what I think it does. I'm a software engineer, so when I talk with colleagues at work some very heavy-duty concepts fly in every direction. Here's how the conversation might go if someone in the office is new, or new to something specific we're talking about, no matter how many of us have PhDs (I used to work somewhere with one non-PhD coder who got teased for being the odd one out):
"I'm having trouble with the effects of R's parallelisation on string interpolation."
"Why don't you convert any vector arguments to lists?"
"I have discovered interpolation is unusual in applying the SIMD parallelisation characteristic of R even to lists."
"That suggests a need to cast either object type to a string within an argument."
Now here's how it goes if this problem arises, say, a week into us familiarising ourselves with R:
"Darn it, R's looping when I put a list in this message."
"Seriously, a list?"
"Just when I thought it couldn't tick me off any more."
"Guess you've gotta turn it into text."
That's what belongs in your writing. Make your characters cognizant of how little it takes for their speech to have the desired effect on their audience. That "forefathers" example actually works, with the right in-universe audience.
(Also, I've avoided profanity because this is SE, but your writing can differ.) 873