I know a fair few people living in flats with no/communal garden space but with pets. How are you meant to take a dog out if you're in self-isolation? Even if you didn't actually walk them, just took them outside, you'd still be touching door handles in the communal hallway. If you have a cat with a litter tray, you still need to take the litter outside fairly urgently.
What would people in flats do about taking bins out? You can't keep a bag of rubbish in the flat for 2 weeks, particularly if it has cat litter or nappies in it. Again, touching communal door handles/bannisters.
Families would find it very difficult to keep apart from each other, particularly with small children in the mix (as anyone who has ever tried to work from home with kids in the house can attest). Especially if there was only one bathroom in the house, or they were the only adult.
What if you live on a farm and have to feed animals?
What if you had a water leak or the boiler broke down during your self-isolation? Or your freezer with all your prepped food broke? Or you had a non-Covid 19 medical emergency like falling downstairs or cutting yourself badly.
It's not realistic to think people could self-isolate properly. Yes, we could cut down on a lot of contact, but I think most people would be pushed to do it completely to the letter. I'm not suggesting for a minute that I wouldn't try, but I think the reality would be a lot harder than it sounds.