Because they can. Peer-review publications are the way people measure the worthiness of academics, so their producers job depend on getting published in prestigious scientific journals, so they put up with it.
The only way this is ever going to chance is if they make the law such that you have to publish in a free to access journal, but since the current mafia has all the money, who do you think has the most lobbyists?
If an open-access journal can become reputable enough, the niche in a field interested in the journal will publish there- period. It doesn't require laws to be passed.
The worthiness of journals, for the purpose of determining how much grant money a publication should count for at least, will be determined by official regulation soon enough if Danish politicians gets their will through.
The only way this is ever going to chance is if they make the law such that you have to publish in a free to access journal, but since the current mafia has all the money, who do you think has the most lobbyists?