All these posts about loaner cars should probably move to another thread, but I think everybody is basically saying the same thing. Commonly, dealers (especially of expensive cars) will offer another car of the same make when you have to leave the car for service, especially if it's a warranty item. It can be a good marketing opportunity for them to have you drive another model, or a newer model, etc. Plus of course it is a big help in customer satisfaction.
Heck, my local Audi dealer maintains a small fleet of cars as overnight loaners, so that they can send interested buyers home with their chosen model for the day. I'm sure that that wins them sales and makes the practice a moneymaker for them.
I would not be at all surprised to see Tesla do both of these things eventually. But right now, all of their production is going to customers. Every car that they produce to be a service loaner is major money off the short-term bottom line at a time when they really need to show profitability and hit their delivery targets. It's a business decision. It's unfortunate, but it's probably the right decision at this juncture.
Later on, they can keep a few loaners available for people having service done, and sell them on as lightly-used certified pre-owned cars so that it's not a long-term money loser. But to get to that point, they need to have the business on solid footing.
I'm willing to cut them some slack on this for awhile, even though it was a big inconvenience to be without my BRAND NEW car for four days to fix items that shouldn't have needed fixing in the first day of my ownership. But my willingness to cut that slack is going to decline a little every day.