One could argue, I suppose, that a person who isn't able to finance their own trip without a sponsor shouldn't really be travelling at all if they cannot be self-sufficient.
I would certainly not argue that! It would effectively stop all but a small percentage of people applying for visitor visas at all, from quite a few countries of the world. The result would be that families would be kept apart, and I would say, totally unnecessarily.
My wife came here from Thailand in 2001. Prior to that, no she did not come here as a visitor, but had she applied to do so, and a pay-for-yourself rule had been in place, well I have to say that as a College Teacher of 11 years standing she earned the equivalent of just £2200 pa! And whilst that was enough to live on in Thailand, there was hardly any ability to save up for airfares and certainly not to pay for her stay in the UK.
Her parents have visited us. They were granted visitor visas, no problem, and duly returned before the expiry date. Both are retired, they both had very good jobs in Thailand ... for example my father-in-law was a Headteacher in a school ... and by Thai standards their Government pensions are good and they live a reasonable life there. But again, absolutely no ability to save sufficient for pay for their "board and keep" whilst visiting the UK.
We are quite prepared to pay deposits should they visit us again, because there is no doubt that they would return prior to the expiry date of their visas.