A journal has editors, some choose to have their journals in print which requires much money, and some journals have sub editors, and assistant editors, and chief editors, and a whole infrastructure which demands much money to be upheld.
If you do not pay for it, well not you precisely, but the professionals who work in that field, then the funds would be taken either away from research or from students. I would rather someone who is in their thirties, and thus able to afford merely £10 for an article, or £30 for a monthly subscription, or however much it is, pay for it, than the students, amongst whom may be many poor ones and amongst whom almost inevitably the next Einstein come from.
There are problems with the system for certain. Personally I think such research should not be beyond a firewall for those who can not afford to pay for it. There is no reason that those who can should not, for if they do not, then the poor will pay.