I think there's a lot of things which make sn obesogenic environment.
I'm quite sure many women from earlier generations would have used ready meals, had they been accessible, but they weren't. Just because you can do it or have to do it does not make cooking from scratch every day fun for everyone. I mostly do cook from scratch (not least because I'm unemployed,) but currently, I am very uninspired by it. i have quite a few homecooked meals in the freezer, but I just can't be bothered lately. Salads at least are easy.
Many women from earlier generations would have driven, had they had that option, too. Mostly they didn't, and they didn't have freezers, either, so food at had to bought little and often.
In my '70s/'80s childhood, we rarely snacked, and being out of town, didn't even have easy access to a corner shop for a packet of Spangles or penny chews.
We didn't have central heating, either, and I suspect sleeping in bedrooms with ice on the inside if the window in winter uses more calories than double glazing and radiators. Housework was harder, too, but I do not want to return to a time of having to boil wash everything manually and stick it through the mangle.
Preservatives were mostly salt, sugar or vinegar, whereas some ingredient lists now are long lists of things which you probably can't buy as a normal supermarket consumer.
There are things governments can do, such as not allowing planning applications for new fast-food outlets within a certain distance of schools. This has been shown to have a positive effect in areas where such rules are applied. Local authorities could also ban junk food advertising at bus stops and in hoardings near schools. Some countries don't allow such ads on TV at times children are like to be watching.
I don't think any single thing has made us fatter, and I don't think any single measure will reverse it, but there are lots of small measures, personal and public, which could add up to improving things. However, from the authorities, such measures usually need planning and funding over different departments and over years. Councils are really struggling for money currently, and local and national government mostly do financial planning only for 4-5 year periods, i.e. one electoral term at a time at most.