There’s your reason why it’s not necessary. They’ve done all that stuff to profit themselves, not to help you get a mortgage. They just want your data.
The banks which actually lend mortgages don’t actually care if you’re in the <1% of the fintech-using population letting some start-up ‘report’ your rent. Lenders have their own criteria and are happily lending to many more people who don’t do that.
I’m sorry to break it to you, but using Credit Ladder is not going to be the thing which gets you over the line with a mortgage application.