[tomatocracy]

Part of the reason for this is that the lender really needs everyone whose name is on the title deeds to sign the mortgage deed and also wants to ensure that none of them are able to claim an equitable interest to prevent a foreclosure. A simple joint mortgage is a relatively quick way and reliable way to ensure this.

The risk if they don't look after their security properly is that the lender ends up owning (say) a 1/3rd joint share in a property which they are not permitted to sell without permission from the other 2 owners and on which they are not entitled to any rent and can't evict the defaulting borrower. There are cases where this has been the result of a defective mortgage arrangement. Understandably, that's not something lenders really want to risk.

It is possible for property to be owned with only one name on the deeds and one name signing the mortgage deed by establishing a trust.

If you want the lender to consider multiple incomes on that basis then you'd need guarantors and you then run into the problem that this probably needs specialist human legal input so it is suddenly not a mass market product.

It's also worth noting that under joint and several liability, the tenants would usually have the right to then sue one another for the relevant shares if the lender went after one of them for the whole amount.