It's the same idea as how public companies' earliest issuance of any potentially-stock-affecting news, has to be through equal-access public channels that investors can subscribe to for equal-timeliness reaction to news (rather than, y'know, Elon Musk's Twitter feed, where Twitter gets to control who sees what when.)
People should be able to follow a citation from one paper to another, without having to pay to do that. Not being able to doesn't just discourage reading papers, it discourages ad-hoc peer review—the precise point of having journals in the first place. (Specifically, it discourages the part of ad-hoc peer-review where you check a paper's citations to see if the claims you find hard to believe, but which are cited, are actually backed up by something you can believe in the cited paper.) When the citation is to a closed-access journal, you have to happen to coincidentally have a subscription to that journal in order to be able to find out what the citation really says. If the citation's source journal is both for-profit and obscure, then you probably don't have that subscription—so the given citation could have been entirely made-up, and you'll never know.
† Journals being aggregators of content also implies that a paper should be able to be published in as many journals as want to carry it, because none of them "own" the resulting content anyway. In that sense, scientific papers would be a lot like news-wire releases: something that any distributor can pay for the right to print a copy of.