Because Muggle Tech Doesn't Do Well With Magic and Magic Isn't Logical
We know from Harry Potter and the Goblet of Fire that Muggle tech doesn't work around Hogwarts, because Hermione mentions it in relation to "bugging" Harry. (that microphones and recorders don't work because of the amount of magic arond) In the same book (at least if memory serves) it's also mentioned that a lot of muggle technologies like electronics don't work/go haywire around magic/magical places, like Hogwarts. So there's no incentive for even a muggle-born wizard to learn much about things they wouldn't be interacting with. Even worse, I might not be ABLE to use certain things reliably, because my magic may interfere with the running of high-end electronics etc. That's not stated in the books mind you, but it seems like a logical possibility. But if I'm going to spend my time in high-magic environments like the Ministry and/or Diagon Ally, why bother? As an example, I could go look up everything I'd need to know about how to live as a tribesman in a 1100s Inuit village... but I'm never going to do that so why bother? I have a lot of other things I need to do!
The second reason Witches might not learn about muggle sciences is that they're not "wired" that way. In Harry Potter and the Sorcerer's Stone one of the "super secure" defenses around the Sorcerer's Stone was a logic problem. Which Hermione describes as "Brilliant" because "A lot of the greatest wizards never had an ounce of logic"! Muggle sciences (biology, physics, engineering, really anything with math) RELY on logic. If A=B and A+1=3, then B=2 is the sort of fundamental thinking required. But if a great wizard doesn't need logic to be a great wizard, that implies logic is not a thing in magic. Which makes a certain amount of sense. By definition magic defies the laws of the natural world as we muggles understand them. It's obviously not a massive deficiency for a wizard to be logical (Hermione herself is proof of that) but it is a way of thinking that would be anathema to people who KNOW that putting a gram of powdered eelspawn into a potion will let them fly if they drink it, but 2 grams will make 30 bright-silver turtles climb out of your cauldron. There's no incentive to learning about logical things in such a society. Indeed, there might even be some sort of harm in learning about them, since you might be tempted to apply the same principles to magical situations where those rules simply don't apply.
Both of these goes without saying that there's no reason to learn about any structural engineering or anything like that, when you can just magic a thing in place. Why bother spending ages doing up blueprints and strength tests and everything else when you can just tac things together with a permanent sticking charm, add some levitation to anything that seems like it might fall, and strengthening solution to anything that seems like it could break? Eyeball-engineering+magic is going to be waaaay quicker than relying on actual physics, with the added bonus that you don't have to worry about some machine going haywire because the magical density of the area happened to exceed the machine's operating parameter!