(I think more networks should be built with default deny egress policies, personally. It would make data exfiltration more difficult, would make ML algorithms monitoring traffic flows have less "noise" to look thru, and would likely encourage some efficiency on the part of dependencies.)
My guess a combo of economic incentives and weak legal protections.
I realize that answer applies to so many issues as to be almost not worth saving, but I think it's still true here.
Way early in my career the "Security auditors" were the cyber security experts that programmed since they were children and actually read and tested the code.
Link saturation would be one of the first things that would come to mind in this situation, and at these speeds QoS would be trivial even for cheap consumer hardware.
I think overall society has rapidly deteriorated in software quality and it is mostly because of the devaluing of software design. No one expects quality from software, everyone "understands there are bugs", and some like to take advantage of that. And so the Overton window gets pushed in the direction of "broken forever good luck holding the bag if you use it" rather than the more realistic "occasionally needs to restart IFF you hit an issue and it takes less than <10 seconds and has minimal data loss".
Wifi connection settings in Windows have a "metered connection" setting, which disables automatically downloading updates. I don't recall exactly when this was introduced, but I had to use it for a year while I was stuck on satellite internet. You can even set data caps and such.
Of course, it's always off by default, and I have no idea if there's any way to provision the connection via enterprise admin to default to on for a particular network (I would assume not) so you'd be stuck hoping everyone that comes in does the right thing.