Because there are shorter and longer hydrocarbons in the oil as well. Most of the longer hydrocarbons can be cracked apart, but the shorter (both liquid and gaseous) and extremely long (e.g. tar, bunker fuel, ...) hydrocarbons will still be present and need to be dealt with.
Plastics are made chaining small molecules into long chains. The property of the plastic deepens on the initial small molecule, how many of them you chain and other stuff.
Also, if the initial molecule has only two "sticky" points and form a linear chain, or has three or more "sticky" points and then form a net of interlocked chains. Or something in between.
Also depending on what is in the initial molecule, the chain may be like a single smooth rope, or have some dangling parts like a Christmas light line.
And you can also mix two or more initial molecules, and there are a few methods to make them "sticky", and ...
In particular, the size and shape of the initial molecule is very important. You want a very specific one to get the properties of each plastic, not a mix random chemical stuff.
Back to oil... Oil has a mix of hydrocarbons of very different sizes and shapes. Some are linear, some have side chains, some have cycles. (This differences are also important if you want to use them as fuel or lubricant.)
To make plastic you must add two "sticky" points to the hydrocarbons, changing some part of them, for example adding oxygen or nitrogen. But before that, you must select the correct length and shape of the hydrocarbon.
To select the length and shape of the hydrocarbon, the fist step is to boil it, and let the vapor condensate at different temperatures. The short hydrocarbons have a low boiling point and the long hydrocarbons have a high boiling point. Then each part is further proceed and used as a different kind of fuel or for different chemical reactions (to make plastic and other stuff).
