Part of the problem here is our culture of excess, and lack of personal responsibility. Excess drinking, excess eating, but we're less inclined than other nationalities (excluding the US) to include exercise in our daily routines.
We don't "need" the gym to exercise, but for many people it's the place they do go to exercise - be it to lift weights, attend a class, swim etc. Some people feel more motivated working out in a group, some people prefer weights to cardio. It's not practical / cost effective / possible for everyone to stock up with all the gym equipment you need to work out at home. So keeping them closed when everything else is opening up IS a problem.
We need more cycle lanes. May roads are simply not safe enough for cyclists. We're now in a situation (at least in my open green space) where we have runners, walkers, cyclists and dog walkers all sharing the same space, with the odd idiot doing wheelies / racing past you / grazing by people for fun.
As for eating out, yes, you can make a healthier choice, but even then, it's not as likely to be as "healthy" as you could make it at home - either because of portion size or because it's not actually as healthy as it may seem. Plus there's often added temptation - oh, I'll have a side of chips with my salad / an alcoholic drink etc.
Like everything in this pandemic, the messaging has been all wrong. Wahaaaay, we're opening pubs, go out and get pissed. But also, lose weight NOW. You can't go to the gym, but you must exercise...etc etc.
I said it's "part" of the problem because there are some people for whom losing weight is difficult, due to health conditions etc. And that is where the medical community can and probably should help out. We should also be mindful of people that are at the opposite end of the spectrum - underweight - and what the "lose weight now" message says to them. If I'd seen this at 16 it probably would have made my ED worse.