Others have already touched on the technical and regulatory issues, but also think of pricing. Would they price is like the US carriers do, or would they price it like a Scandinavian carrier?

If they want customers all over the world, they'd need to match the cheapest offering, as it's hard to argue that the service for some reason is more expensive in the US. I tried finding the price for a US cell phone plan, I can't actually do it. AT&T is impossible to navigate and always seems to bundle a phone. Mint Mobile tries to push 3, 6 or 12 months, which isn't even legal and the plans still come in at $40 for what is a $25 plan here in Denmark (with no additional fees).

If Starlink launched a world view subscription, at competitive prices they'd immediately crash the US cell service market.

For travellers, they only need to match other global esims, which are quite expensive