Sure, many of the decisions in the current system are 'impure' in some way. Everybody has a price, everybody makes mistakes, everybody is uninformed about things. The big difference is the decisions still come from the elected lawmakers. They're often wrong, and even more often just guessing, but in the very least the system is transparent, we know where the power resides.

In the case of a randomized group, that power isn't with the group, it's with the people who educate them on the matters and who frame the issue. There are a million ways to present any matter, and none of them are objective. You're putting the power to influence and even determine a nation's decisions in the hands of the experts that inform them, rather than the randomly selected people. It would become very efficient to buy those people instead of the voters.

The problem of the matter isn't whether it would function worse. The problem is that it would function less transparently. It would ostensibly be a fair and completely unbiased system, but that could be manipulated behind the scenes in far more insidious ways. That's very dangerous. Personally, I would rather have a more unfair but more transparent system.