I learned about the equilibrium constant. Now, I've seen that the equilibrium constant of burning is extremely small $(K \ll 1)$. here, I have a question. you see, $K$ is still NOT 0, which means that the forward reactions happen at least a tiny bit. Then, shouldn't we see some parts of anything burning at least a little bit?
shouldn't I be able to observe this tiny bit of combustion happening at room temperature? if $K$ is extremely small but not 0, I don't understand it.

In general, I've always wondered why we don't see small amounts of reactions happening spontaneously since the equilibrium constant isn't 0.

There's the concept of a "threshold energy", but it seems a bit arbitrary and not well-defined.
why don't we see anything burning continuously? I know this question might be stupid, but I really want to know.

You are correct that the equilibrium constant for the combustion reaction is very small, typically in the range of $10^{-14}$ to $10^{-30}$ depending on the specific reaction. This means that at room temperature and atmospheric pressure, the forward reaction (where reactants are converted to products) is many orders of magnitude slower