It's because they're good speeds in a lot of places that couldn't get good speeds before. It's also great for mobile work sites, i.e. construction sites, drilling camps, other b2b service businesses where a bunch of portacoms rock up to a site. Anywhere it's mildly hilly you can't actually assume you'll get a signal outside of town but a satellite dish basically guarantees that. Even if you can guarantee your in a spot long term the upfront cost of fibre or a tower may not balance out as cheaper than just eating the higher bandwidth costs.

It's also worth remembering that in a lot of places with low density it isn't appealing for competitors to build out to, so there's a lot of markets where it's a no brainer to switch from the local monopoly to starlink because the price was already inflated and it was worse service.