Why? It seems to me to be much cheaper than a building. Musk's aim is for the cost of one Starship launch to be under $10 million in the long run. 50 rocket launches would be less than half a billion dollars. Can you get a datacenter built on the ground for half a billion dollars?
The comparison isn't half a billion for a datacenter, it's half a billion for a big shed (and some solar panels). The problem of space based data centers is that the only effort they save you is building a big shed to put the GPUs in (and potentially cheaper power at the cost of more expensive cooling)
You haven't built a data center for half a billion mate, you've just launched rockets. You still have to build the data centre! Only not from bricks, it's going to be complex modular units made of radiators and solar panels and you have to buy the gpus too, and the whole thing will be bombarded with cosmic background radiation but you can't replace or repair any part of it.
"If you give me 5 years and everything goes perfect, I can give you a system that's 9% worse than what you already have", you don't have a great investor pitch.