The EU is so insidious that sometimes it’s hard to see what they are doing. Especially so when your own government and civil servants keep up the pretence of sovereignty.
However it isn’t individual laws or dictate or the prolific spending or the unelected officials who rubber stamp their followers into positions of power or their demand for ever closer ties or their demand for a single currency or their drive for a unified EU army, or their taking away of sovereign powers such as foreign policy and diplomacy it is not one single thing it is a combination of all of it.
Some people articulate that as lightbulbs, some as straight bananas, for others it’s the waste of moving the EU parliament between Brussels and Strasbourg. You will find a hundred people with different one off reasons that they can say but it is the overall effect that is the reason for Brexit.
If remainers insist on doing a zacmaster of ‘give me one reason’ and the that one reason is belittled and dismissed and conveniently forgotten the next time they ask for ‘just one reason’.
So I would suggest that if anyone asks that question now they haven’t been listening for the past three years, are deliberately ignoring it or simply trying to sound pompous. Perhaps all three.