Conversion is only the first part of the mash, converting starch to sugar, further time is required to get the desired types of sugar in the wort. A short mash time will give more large, complex and difficult to ferment sugars, a long mash time will give simpler, more fermentable sugars. Short mash will give a fuller, lower Abv beer, a longer mash will give a drier, higher Abv beer.
A good analogy is, felling trees converts them into wood that can be used for fuel. However, further time chopping is required to remove the branches, cut the trunks into manageable logs. Then, even more time is required to split those logs into wood that will fit into a fireplace.