The problem is that house builders are cheats and liars. Yes, really!
They do all sorts of tricks to try to con banks.
Buy this house and we'll pay your deposit - that just means buy this house which we've added on 10% to the asking price. We'll cut the price down and only the bank loses out.
Buy this house and we'll give you 5K cash back - That means they've added 5K to the price.
Buy this house and we'll give you your appliances - Same thing
They used the vendor gifted deposit thing until banks got so fed up with it all they want huge deposits on new builds in general.
The latest scam is with Hitachi capital where they're going to say you only need a 5% deposit, and your parents guarantee the rest. Instead what really happens is they credit score your parents to make sure they could turn an unsecured loan into a secured one if need be, take 20% from your parents, charge them a very bad mortgage rate, charge you a very bad mortgage rate too.