What you describe is much more than continuous voting. It is also direct democracy, or at least liquid democracy.
If your question is not rhetorical, the first answer which comes to mind is that we did not have online voting. Online voting is still extremely rarely used by governments (below 1%).
If you are describing a form of direct democracy, then I would say this is rare because real direct democracy is highly inefficient (I described disadvantages in an introduction to liquid governance).
If you are describing "liquid democracy", then I would say it has been considered by many, just not deployed yet. So if are asking why liquid democracy has not been deployed, I would say this is because implementing liquid governance is much more technically challenging than implementing representative democracy. Liquid governance basically requires electronic voting, and even electronic representative democracy is still marginal. The page linked above also discusses the implementation status of liquid governance.