Many are just not that diligent with proper dental hygiene. Interdental brushes/ superfloss are used only occasionally if at all and not every day. There are people that brush for 2 minutes and call it a day, because they heard it's enough in some advert or because the electric toothbrush stops. Well, it turns out you need a lot longer than that and a reasonable technique if you want to keep your teeth clean and healthy.
The acidic, sugary drinks and food don't help at all. Drinking mineral water (no sugar, no extra acids) or to wash the mouth with regular water or a low concentration sodium bicarbonate / baking soda solution to balance pH after eating/ drinking something else would probably help. Of course, if not dissolved completely, the baking soda could act as an abrasive which wouldn't be that great for tooth health so probably just use regular water.
You don't need to invest much money to keep good oral health, it certainly is much cheaper to fix the problems that will arise if you don't, if they can be fixed at all. It however does cost effort/ time.