It's an outside event, but it requires a sizeable number of people being funnelled in and out through confined spaces, sharing cramped facilities, etc. Also relying on fans behaving themselves and following the rules, every club has it's share of zoomers that will ruin it for the majority. As wellwell91 says, how do you control what happens before and after games, travel to and from the stadium, milling around before going in, etc? Policing and managing all of this costs money that clubs wouldn't be spending in normal times. Then the practicalities of choosing what fans get in. And say we find a solution that works for all parties, why do I need to stick to the rule of 6/2 households when there are literally hundreds of different households all at the same event sharing the same facilities? Fuck this, I'm doing my own thing because if it's okay for football then why isn't it okay for me?
Playing devil's advocate here, but the point is that it's a can of worms that could exacerbate things. I know everyone has their own view on how things have been handled, but the fact is that we have rising numbers in the country just now (almost certainly linked to schools/universities being back, but not exclusively because of). Letting people back into stadiums when the death toll is slowly creeping up and you can't even go and visit your granny would not be good from either a PR or public health perspective. That isn't going to change until it is clear that we had the situation under control. Other countries having fans back in stadium may work for their situation, it doesn't mean it would work for us at the moment or that it is even the right thing to do.