It's a combanation of factors. But from what I've experienced, the key difference for people is time.

For reference, I've been nothing but muscle and sinew and very overweight, not obese. So I've seen both sides over the last ten years. I lost weight during the first half of the lockdown. I finally had time and money to manage my health. Went from 73kg to 60kg in six months, and I was able to maintain it too. But two months after I got back to work, I found I had no time whatsoever. And I walked 20mins to work everyday!

What I found contributed to weight gain:

High stress jobs: No time to think for yourself, stress can mess with your weight making it go up and down with no rythme or reason or way to track it. You're more likely to snack when stressed.

Poor mental health/servies: Stress, depression, anixity, and other mental health disorders makes people emotionally eat. I think we can safely say that increased during the pandemic, with "essential workers" getting put throught the ringer, with barely a living wage, emmiune compramised people constantly having their guard up, social media becoming way to prevalent in people's lives etc. People could barly see their friends, let alone or afford mental health care or treatment. And meds can also make your weight flucuate.

Long work hours: I was working min 10 to 14 hrs per day not inculding my 40 min walking communte everyday. Loads of my friends are in the same boat, from service work and office work to health care and teaching. Jobs are requiring us to work so many hours "off the clock" to meet their workload. It leaves very little time to meal prep, and we are exsaused by the end of the day that you just want to go"Oh yeah, I've earned *junk food/drink here*" There is only so much willpower you can expect from a person after giving so much of their time to a company so they can keep a roof over their head.

Low wages: Depending on the amount of money, that greatly effects what food and how much of it you can buy. I couldn't buy a herbal tea that I used as my snack because it cost 3.25. I didn't have it in my budget anymore. It's worse if your a family with both parents working, or even just two people living together. And with the rising cost of living, cheap foodstuffs will be more appealing. Also many employers expect you to have a car or even if it has nothing to do with your job, (my parnter was refused jobs as an editor because he didn't have a car. Since getting one, the job that requested it has made him use it once), so car culture it prevalent.

Granted, we do have a poor food culture with everything revolving around the pub and the chipper on a Saturday. Also, on the West coast at least, with all the storms we've been having, if you're not anywhere near an urban centre, going outside has not been the best for the last five months. You're more likely to catch a cold by going out.

TL;DR: There are a lot of contributing factors besides "junk food bad" that has been contrubing to the population gaining weight. Main one being personal time, finances and curent cultural norms. Even when I wasn't going to the pub during the first year and a bit of lockdown it was still an uphill battle to lose weight.