The difference between a home mortgage and a record label's advance is that the value of the home being mortgaged is
relatively stable. If I borrow $450K to purchase a $500K house, and then I can't make my payments, I am likely to still have a house with a market value close to $450K. (OK, in many parts of the US today this is not true, but at least that was the bank's expectation when they authorized the mortgage.)
If a new band approaches a record label for a contract, the label knows how much it will cost to produce and promote their album, but given the fickleness of the public taste, they don't know whether that album will sell ten thousand copies or ten million. And after the band has sold ten million albums, it will have a lot more negotiating leverage in the next round of contract negotiations.
So the record label has every incentive to lock the band into a contract that grabs as much as possible for the label. Bands that only sell a modest number of copies get screwed by this arrangement, but if the band members only expected to sell a modest number of copies of their album, they wouldn't be seeking a deal with a major record label to begin with, right?