A major (perhaps even the primary) consumer of the current ratings is the institutions of various governments providing funding for the scientists and the institutions; who are not really able to assess the quality independently (especially due to various perverse incentives making many such institutions likely to intentionally misassess the quality even if they had the diverse competence required) and also definitely not willing to trust the beneficiary institutions/scientists to assess the quality themselves.
And apparently they're willing to pay quite some overhead just to have a separate third party gatekeeper perform that function for them.
This is often not true, and if it was true it would be backwards.
Funding bodies also employ academics who are able to judge the quality of proposals or they solicit feedback from experts. You don't just write "I've got a bunch of first author papers in Science so you should fund my grant." This is important because if the only thing that funding agencies use is institutional reputation then we end up with an even larger percentage of funding only going to entrenched academics at a very small set of institutions.
I'm seeing KPIs for grants (and even for the whole programs of grants) as, for example, quantity of publications in top quartile impact factor according to e.g. Scopus and Web of Science.
They do not want to evaluate each publication separately or pay experts for a separate review of each publication (if they would, that could be a source of funding proper peer review instead of it being done by random volunteers) but rather defer to the existing ratings of the publication venues. They need a simple objectively measurable quantitative metric, not a complex qualitative one.
And apparently they're willing to pay quite some overhead just to have a separate third party gatekeeper perform that function for them.