Plus, about half those old books had information a modern person generally hopes to never use, even if they do still make things. Much as I love living history, I don't want to mill lumber by hand or do anything that involves the phrase "Measure out the white lead and mix it into the...".
There's lots of stuff that would be interesting and potentially useful, but information these days seems to spread based on immediate usefulness (Except math. Everyone just accepts people all should learn a pretty large amount even though only some of them will ever use it), and people don't really think about uncommon scenarios.
But now it seems like we're right on the edge where even modern practical things are being forgotten, and "I don't know how to cook" is starting to literally mean "I doordash every day and couldn't make a pot of spaghetti to save my life, and I think an impact driver must be a porn star name".
Percents, fractions, what the word "average" means...
Most people almost never do these things themselves, unless they like the process of planning things on paper without tech, or they do these things so often you'd be seriously slowed down without being able to them mentally.
Everyone should learn math, because it lets you do the things that need it, and because for a lot of people, the mathematical beauty seems to be the motivating factor to do those things in the first place, and those things tend to pay well and might save the planet.
The problem is when people act like we will be balancing checkbooks, picking stocks by hand, and adding fractions on paper every day.
People hear that and think it's useless because they are confident they won't be doing those things, because... I've never seen anyone use calculus in my entire life. I'm not sure I've seen algebra either.
I've heard about geometry, but mostly extremely basic things, or for things that would be easy to do in CAD.
Honestly I'd probably make a FreeCAD sketch instead of doing the math by hand if I was going to find the height of a tree with the Pythagorean theorem, being able to actually see it all would be an extra confirmation I did it right and the measurements were at least approximately good, and I'd have something to screenshot to prove the method works, to people who know even less math than me.
Fractions mostly show up when dealing with material that is sized in fractions, and half the time they're not really fractions, just nominal names for things, And you'll need to look up or measure the real size anyway.
It's not like people are hitting brick walls and thinking "Oh well, can't do this project, the math is too hard, shoulda stayed in school", it's more like people say "This doesn't pay well, maybe if learned math I'd have a totally different job and be doing totally different stuff I can't even imagine".
It's not like a hammer that you know exactly when you need it even if you don't have it.
Plus, scientists get things wrong sometimes, and there's tons of flaws in papers, so even just for pure understanding the world you need a lot more than math to actually be able to read a scientific paper or a political survey, and without all the other stuff the math looks less useful than it is.
I use percentages constantly to analyze choices every single day, in my personal life... Not doing so implies lack of due care and consideration.... I have to ere on the side of people not being lazy, willfully ignorant and unthinking, right? Ere go most people must use percentages constantly as I do.
Angles? Telling someone what angle something is, is something I do regularly... It's a right angle, or 15 degrees or whatever...
I could go on but really...