What is missing from the transaction
In an ordinary purchase a great deal rests on the seller's replies: what work has been done, what consents were obtained, whether anyone has ever complained, what the arrangements are with the neighbours. A repossessing lender can answer none of it.
The usual contractual protections are also commonly limited. Lender sale contracts are typically drafted to exclude the warranties a private seller gives, which is a commercial position rather than an unreasonable one — but it is a position you are accepting.