The key issue is that we now need a reasonable floor - which usually means indoor, or at least covered. And that brings all the extras: chairs, changing, water, etc. (BYO)I like your rose-tinted glasses!
Sports hall: can be expensive to hire. Often has a strict closing time (usually 10pm where I live). Often has a shoes code e.g. no outdoor shoes, non-marking soles only and definitely no high heels! A lot of competition from other groups.
Hidden costs: obviously organisers’ time! But also advertising, insurance (venues often require organisers to have their own), heating and lighting - sometimes there’s a meter and sometimes there’s an additional fee, sound equipment and storage- a cheap system won’t last very long and an expensive system can’t just sit in someone’s car all week. Stationery: most venues require you to have a sign-in sheet for fire safety. A lock box for money. These days you also need to bring disinfectant wipes and hand sanitizer.
You can’t run a milonga without either a bar or a refreshments table. A practica can be run without either but it’s best to have some water on hand.
I also think that if tango can’t sustain itself then it’s going to die out. So it’s better to have a few for-profit organisers than to have many hobbyists. Burn-out is very high among voluntary organisers.
Whilst I've spotted a few videos from Argentina during, or probably with some relaxation post-lockdown, of milongas in the park or similar, it's become less practical in Europe (depends on country) - even if you take a floor! First, there's the weather. Then there's the developing issue of private 'public' space, e.g. some covered area adjacent to some building. It's owned by the building's owners and you have limited access. You may get permisdion to use it if it's considered 'art' or 'cultural'. Bylaws can affect use of park space (no picnicking, no ball games, etc). Can't even access some bandstand (protected against vandalism). About the only place I can think of might be a public beach (apparently sand is ok to dance on - barefoot or flats; no high heels!)
You're right. Unless someone owns a barn with a suitable floor and makes it available, it's going to cost. As you said, there's liability insurance and the rest unless you go for an illegal rave or it really is a small 'private' gathering.