Business relationships with carriers, and similarity to iOS which disables tethering by carrier settings. Carriers strongly objected to tethering and sometimes still do because they would prefer every device register individually with the carrier on a paying plan. This has changed in subsequent years but carriers did and still do lock down their networks to prevent devices from connecting or billing different devices at different rates. Generally though it’s a profit motive, followed by a network protection measure.
For the carriers it's not just about wanting to squeeze customers for more revenue (though it is also that).
Their networks are often provisioned under the assumption of super bursty traffic, where not all phones will be pulling at throughputs all the time. Laptop users will tend to use high throughput more consistently, and so be bad for their business model on the cost side.
