Because the journals existed as massive, financially powerful entities. There were negotiations over open access arrangements a few years back which led to things like “gold open access”, which involves papers being free to read, but costing a heckton for the researcher’s in “Article Processing Charges”. This happened because the journals effectively argued that “even though we’re functionally useless in the modern day, and don’t even provide services like copyediting or typesetting support for researchers, you can’t just make research actually be fully open, because then we would no longer be able to be absurdly profitable. Won’t someone think of the profits?!”. And then their influence meant the open access agreements were half baked and insufficient.

However, there is a continuing movement that is pushing for actual open access — “Diamond Open Access” doesn’t charge either the researchers or the readers of papers. It’s still small, relatively, but it’s growing, especially in the global South or amongst independent researchers who can’t afford absurd Article Processing Charges. Profit driven journals have prestige on their side, but I reckon that Diamond Open Access will continue to grow as research funding becomes more scarce relative to the amount of research being done.

“The diamond model has been especially successful in Latin America-based journals (95% of OA journals[1]) following the emergence of large publicly supported platforms, such as SciELO and Redalyc. However, Diamond OA journals are under-represented in the major scholarly databases, such as Web of Science and Scopus. It is also noteworthy, that high-income countries “have the highest share of authorship in every domain and type of journal, except for diamond journals in the social sciences and humanities”.”

(Source: the linked Wikipedia page)