Because if you drink 500ml of a 2L bottle and put the cap back on, the rest goes flat.
Are you screwing the cap on all the way?
IME, not only no, but they will yell at the housemate who does for putting it on too tight.
It used to drive me mental when people would complain about how tight I’d screw the cap on. What do you even think the point of that thing is?!
Yes, it probably won’t be completely flat after half a liter but the fizzyness will be noticeably weaker and will only get worse the more you drink, especially if you don’t get to it soon. Two liters are ok if you’re sharing them with multiple people who will finish it right away or the next day but they suck for one person unless you drink a ridiculous amount of soda.
I think you’re just shaking it too much. If you are gentle to avoid losing the carbonation and put the cap on tight, it can last for days.