The Constitution is the same way. It's a check on democracy, but it's also democratic: "we agreed to do it this way, and if we want to change a rule we have to reach a new agreement through a cool-headed process."
In this case, the Constitution creates limits on what kinds of things may be resolved by the executive branch, and what must be done by the courts. That allocation of power between branches is not something we want to leave to whatever people think is a good idea on any given day. As to why we look to 18th century England: the founders thought that English courts worked pretty well, so they basically forked English common law. So sometimes, to understand our own law we look to what the English courts were doing before we made our own fork.
Sometimes you like the end result, sometimes you don't. Sometimes originalism means that a terrorist who is a U.S. citizen has to get a jury trial in an Article III court, even if the prevailing public opinion is that those people should be tried in military tribunals.