That's because we are too tired to argue.
He would show some numbers and make completely wrong conclusions. There was no way I could get him to try to listen to me. Whereas back at school, he was never arguing with me about maths given that he had bad grades and I didn't.
So somehow I really feel like there is this "authority" thing missing. Now that he is an adult, he feels like his opinion about everything is valid.
When you were at school, you were both part of the same social hierarchy. On the topic of math proficiency (general smartness, really), it was easy for him to perform a mental check and see that you were above him in that aspect (even if he was above you in other aspects), and so he should listen to you.
But then you probably went your own way, essentially getting out of his way with him continuing to advance in that traditional subculture. So now he feels he's earned being considered closer to the top of the hierarchy, even though it's no longer a pan-society hierarchy!
And then he also cannot understand where to slot you into his mental model. So you just get written off as an outside attacking force to be opposed.
This also explains why they continue calling themselves "conservatives" despite gleefully working to destroy our institutions and standing in the world - what they want "conserved" is their perceived prominence of the subcultures they bought into.
I once heard flat-earthers in a train. One was saying "It's like school, e.g. physics: they purposely make it such that we don't understand it to confuse us. Because we're not worse than anybody else, so we should understand, right? If nobody understands, that's because they bullshit us!". I really wanted to say "counter-example: I could understand physics at school :-)".
At its best, "Science" doesn't claim the force between two objects is proportional to the product of their masses. It merely observes that this has been the case in all known, well-documented instances, and attempts to find exceptions have failed, so that this "law" is valid and reliable, and expected to remain so forever.
We use the term "fact" very casually, and eagerly. "The Dow is up 20% year-over-year, unemployment is under 4%, and the dollar is strong. The economy is strong. That's a fact!" Not only is that not a fact, it's not even a fact that unemployment is under 4%. That's (presumably...) just the U3 number as published by the Department of Labor.
My point is: the best "facts" still require a certain amount faith in the data sources around us. That's fine. I don't wish to visit every country on Earth to ensure that it exists, and observe the weighing of the Kilogram in Paris. (I mean, unless I can take time to sightsee a bit.)
There has been a genuine breakdown in that faith in authority in US culture. Maybe Ukraine attacked Russia first. Maybe poor people are inherently lazier than rich people. Maybe the Earth is flat.
I believe this breakdown has been intentionally orchestrated and groomed and fed by nefarious people like Rupert Murdoch, but that's just an opinion. Based on things I like to call "facts".