It's more concentrated power. The opportunity for more corruption. Sure, they could be philosopher kings at first but having the control means someone can have the control corruptly.

Why does that mean it cannot be accounted for democratically?

I don't necessarily believe all small businesses are fine, but their interests compete with each other, and they're small, by definition. And we already have regulations that apply to all businesses, there is democratic control in some sense. So I'm not worried about how the corruption of one small business owner would warp society or national interest.

Nothing is static, they will eventually grow into monopoly and corruption.

I agree with this premise and then not the conclusion. Inevitably, all behemoths were once small businesses. But is the correct intervention to stop the small businesses from forming in the first place, or to prevent the ones that get big from utilizing that size in an asocial way? You could socialize businesses of a certain size, for example. You could set rules for worker-elected board members, or whatever.

The correct path is to avoid the problem entirely via Socialism.