In theory it "should" be much easier to build on earth, but in some ways it's just different challenges. On earth you're forced to deal with those pesky government things and in the Sahara not a lot of them are exactly reliable or good-faith actors. Then there's night time. So out of the gate, you're dealing with needing massive power storage for the night time.
So you invest $5b into a solar farm and data center outside of Tunis and 5 years after you finish construction a popular uprising topples the government and now you're dealing with new management? Nah, nobody is going to do that. And who's going to work there? How are you going to get data out of there? You're going to end up using satellite comms anyway. It's not 1953 any more and (thankfully) nobody is in a position to "Operation Ajax" your popular uprising when that happens too. I mean, maybe, but yeah, I would not do it.
Even in relatively stable places like the US or the EU, let's say you bought some random parcel of land in the New Mexico or West Texas desert region. Or even in Southern Spain or something. Even if you get the land cheap, with relatively easy fiber access (doubtful, but whatever), you're still beholden to the communities there. You think they have spare water to cool your facility? How are the schools for the kids of the engineers working there? You think that people are going to be head-over heels in love with Amarillo or Extremadura? The land is cheap because people don't want to live in these places, so picky people are going to steer clear. And at the end of the day, you're still going to have to get everything permitted, approved, stamped 80 times, and the project will grind on for months.
No, space is an end run around dealing with bureaucracy and politics. It's space. There's basically nobody to tell you "no" up there. You can park the satellites in a sun synchronous halo that lives about on the terminator, and just pull in power constantly and radiate directly away from the sun in the other direction. It's going to be expensive, it's going to be technically challenging but we will do it. Also, think about the California high-speed rail stuff. If you try to build on earth you're going to be permitted and social-media'd to death anywhere on earth you decide to build one of these. For better or worse people hate Elon. I mean, I understand it, he's kind of insufferable and his dalliance with politics was a bit of a disaster (seriously, USAID cuts are killing people), but he's certainly no moron and I do not think he's entirely un-selfaware. He knows that people aren't going to let him build these wherever he wants. You're going to have to ask for permission thousands of times, there's going to be social media campaigns to stop him, literally any screw up (his fault or not) is going to be loudly shouted to everyone. If he decides he wants to expand his facility, that's more permits, more restrictions, more permissions.
So they'll go to the place where they do not need to ask anyone. Initially that was red states or at least relatively "non-hostile" states like where Tennessee, but even there people will squawk about it. I don't mean to say "squawk" to dismiss those folks, well, maybe I do, but I just think it's a bit silly in the context of us burning gazillions of gallons to bomb the Iranians. Nobody is going to do a damn thing about the climate or anything right now, and stopping data centers from getting built feels like stepping over dollars to pick up pennies, but I digress.
Anyway, space has none of those problems. Indeed, the problems are almost all technical. The technicians and engineers can live in California, or work remotely from anywhere really, and you won't have to deal with increasingly well funded and clever NIMBYs. The real challenge is going to be finding optimal launch sites for this stuff. Hilariously, my neck of the woods up here in Alaska is uniquely suited to launch into inclinations that would allow for constant sunlight. It's what, 98 degrees inclination for an SSO? So you can launch launch north out of Poker Flat and south out of Cape Chiniak. Though we don't have the infrastructure up here to support that out of Poker Flat yet. And nobody will squawk too loudly about it up here. I think those lunatics trying to slingshot satellites into space are trying to launch out of Adak too, so, hypothetically, that's an option as well other than the logistics of getting vehicles up here.
Anyway, this has turned into a bit of a book report, but these companies are not optimizing for cost savings right now, they're optimizing to avoid people telling them no.