Why do some chemical reactions require many steps?
The number of steps required to model a reaction really depends on what time scales you are interested in looking at. Let's take the process of burning natural gas with air. If you only care about the final products of the combustion after a really long time, like let's say hours, then a single step reaction would be a perfectly good model. In fact, in situations where you only care about the global heat release and maybe the speed of the reaction at one particular condition, single-step or few-step reactions are just fine. 

But let's say you are really interested in what's happening. When we look at a global reaction, let's say:

$$ O_2 + 2H_2 \rightarrow 2H_2 O $$

We'll assume this is in air, but the nitrogen doesn't really react. Nice and simple, right? Well, for the reactions to occur, you need some $O_2$ molecules to collide with $H_2$ molecules. But it's not enough that they collide. They need to collide with enough energy to break the bonds between them. And it's possible that one of the molecules breaks at lower energy than the the other. When they break, they break into 2 O's or 2 H'2. So our simple reaction really starts out as:

$$ O_2 + M \rightarrow 2 O + M $$
and 
$$ H_2 + M \rightarrow 2 H + M $$

where $M$ is any other molecule. Okay, so now we've got a mixture of $H$, $O$, $H_2$, $O_2$, and $N_2$ floating around and bumping into each other. If they bump together with enough energy, we can start forming other things. Things like $HO_2$, $OH$, $H_2O_2$ and of course, $H_2O$. 

Because these reactions rely on collisions, you need to form some things before you can form other things. So before an $O$ and an $H$ can collide to form $OH$, the $O_2$ and $H_2$ have to collide with other things and break apart. So there's finite time scales with all of the reactions, and they tend to cascade through a series of reactions before reaching the final product. If you are looking at time scales similar to those of the reactions, then these details really matter. 

Hydrogen reactions are super simple. Imagine how complex organic molecules are. Even things like methane or propane have hundreds or thousands of steps to go from the complex molecule through all of the radicals to the final species. And the time scales the reactions occur over are all based on how frequently the collisions between them occur, and how much energy those collisions have.