Because you already pay for it when you buy your ship. You Anaconda doesn't cost 150.000.000 but 50.000.00 and 100.000.000 go directly to the insurance company.
Aaaand that is totally wrong.
If I already paid insurance rebuy upfront then it means I don't have to carry rebuy money on me. So when my ship gets destroyed I get replacement just like that, right? So why there is a rebuy cost if I already paid that upfront? Smells like scam.
And secondly - if it's true - how would you consider multiple deaths issue with a single, upfront payment? If Anaconda costs 50M and 100M went to the insurance company it would mean I can get replacement ship only twice. But what happens when I blow up Anaconda for the 3rd time? Company will give me 3rd ship because they are nice?
The most probable answer to OPs question is the most simple - insurance company doesn't work with ship insurances only, it's just a part of their bigger business. And while space ship part may be deficient they will cover those loses from other, more profitable sources (like multi million pop singer leg insurance fee). Overall, not every piece of business must be profitable if it serve a greater purpose. If the company as a whole is profitable then it's good.