Obesity is a very complex issue! But medications/health problems aside, the ('normal') majority of people could do with educating themselves and taking responsibility for what and how much they put in their bodies....
Most people do not realise that sugar has many different names. That tin of whatever you pick up that says 'no added sugar'? Check it. It might still contain one or more of the other sixty versions of sugar. And it's literally in EVERYTHING! If you've ever tried to avoid the stuff, you know how difficult it is to escape the bloody drug. (It's in things you'd never expect to have it, like bacon! Bacon! for crying out loud). It is a drug. Just as addictive as heroine. It's added to our food and we, like a herd of sheep, mindlessly consume it. The more we consume, the more used to the taste we get, the more we crave.... the more profit manufacturers continue to make.
Do people read ingredients? Do they teach their children to read ingredients? Most don't bother.
A sugar tax would simply make food more expensive. I doubt it'd reduce the problem. Those that can't afford whole foods for one reason or another (money or time starved) will switch to cheaper versions of the same crap. Instead of one bread in a bag, they'll buy the even-worse-for-them version of the same bread in a bag that even mold won't eat.
As a society we've gotten so used to eating processed-for-us sweet-tasting stuff that the truly whole products are not deemed tasty anymore. We want more of the easy, quick, delicious! stuff, and we want to eat until we're 'full'. It's what we teach our children, too.... So it perpetuates.
Until our 'normal' is changed, we're unlikely to see improvement.
Sugar=delicious - normal
Sweet everything, even things meant to be savoury - normal
Eating until 'full'/'stuffed' - normal
90% of your meal being starch - normal
Ketchup=serving of vegetables - normal
Chasing every meal with pudding - normal
Drinking fruit juice (and often this is just water+sugar+colouring!) - normal
Treating yourself=eating something sugary - normal
Anything green=disgusting or tasteless - normal
Not having to cook your food or physically 'work for it' in any way - normal
Never actually feeling hungry - normal
We've stopped chasing the mammoth. We've gotten used to eating sugary spongy food and consider it delicious. We've even stopped chewing properly, because we can easily avoid having to do so.