Earlier quoted context omitted.
Because voting is just a small part of Democracy. Democracies need strong and independent, judicial, law making and executive branches. Voting itself is an averaging process, and you get the average of what ordinary citizen wants. It is hard to make people want good things for themselves without proactive investments in education and developing a population with scientific temper.
Separation between the executive, legislative, and judicial branches of government appears to be a good practice. However, the UK doesn't really have that: Parliament controls everything. And they seem to have a fairly stable and functional democracy.
We do have a stable and functional democracy, though it's rather brittle against a bad-faith executive. One prominent theory is the 'good chap' model of government, which holds basically that the system is set up such that 'reasonable chaps like us' can govern well and with few constraints, but the flipside is that Johnson's impact was limited by his government's lack of competence, not by institutional constraints. That might be liberating or terrifying, depending on your views about good government.