Because the VCs are funding the startup on extremely favorable terms?
If the startup fails, the founders can just walk away. They are not personally liable for anything. They can (and often do) subsequently form another startup, often funded by the very same VCs who funded the one that just failed!
If, OTOH, you fail to pay your mortgage, the bank takes your house. And they make hard for you to get another mortgage from any bank by reporting the foreclosure to credit ratings agencies.
You absolutely can keep the equity (and surplus) for yourself… but you will need to personally guarantee the loan. You may need to declare bankruptcy if the startup fails, and all that entails.
VCs are happy to throw away money on 99 failed startups precisely because they are entitled to the continued surplus from 1 successful startup. Banks are happy to make failure to pay extremely unpleasant for you because they are not entitled to any surplus from business loans which lead to successful outcomes.
