Because damn rules.
In good old times if a student told teacher to fuck off a teacher would've given a student a cuff on the nape and the conflict would've been resolved.
These days? Nope. You're not allowed to touch children. You're not allowed to yell at children. You can call parents at school and have another "he didn't do nothing" conversation.
So what you do? You introduce zero tolerance rules. You call the police. You turn school into prison where the same set of extremely strict rules is applied to each and every student.
And all of that could have been prevented by just one gentle slap!
It wasn't a joke either, I remember a guy showing me his butt with a big nasty purple bruise on it the shape of a board. I got paddled once but the teacher took it pretty easy and I just had a little red. He was a weird old guy and I think he mostly liked hitting bent over young boys with boards.
They could pretty much do it for any reason and some kids not popular with teachers were paddled pretty regularly. Shortly after my time this was discontinued and I was not at all sorry to see the practice go, but now we have gone too far the other way in my opinion. Calling the police on kids for minor infractions is way over the top. But some semblance of order has to be enforced or no one can learn.
I feel like you learned more about life back then (maybe in sort of a "Lord Of The Flies" way to be fair). Most everyone could do what they wanted after school without worrying about stranger danger and having adults hovering over them the whole time and there was a lot of freedom to explore. If you fucked up though, you got paddled and you might get beat up by a fellow kid without a lot of recourse to authorities. It was just a rougher time, but more free as well which probably sort of describes the history of humanity also come to think of it.
I don't know which is worse, then or now. It seems kids were more self confident and self reliant and less neurotic back then. But there were a lot more physical fights and odd kids had a really tough time and kids who came from rough situations often really got beat down by the system. Literally.
There's a pretty wide spectrum between: "Can't discipline kids that act up" and "Beating kids"
Kids need boundaries. It's literally what they spend a good portion of their time testing for. If you aren't allowed to provide boundaries for them, then they will flounder, which turns into a self-reinforcing cycle.
I've heard from some teacher acquaintances that it's highly discouraged because the parents tend to complain a lot and the kids view it as a free vacation from school.
It sounds like "try to ignore them" is basically the only recourse teachers have in some districts against disruptive or worse students.
I think these punishments are commonly decided by administrators, not teachers. The teacher just reports them up the chain for misbehavior.
Suspension and expulsion are becoming embroiled in the anti-racism crusade due to some high-profile incidents, so blunt laws are being written to outlaw them altogether.