Cost- it is nearly always cheaper to purchase a custom mold base and the accompanying components. Why waste your time on the "grunt" work, when the profits are in the mold making? Of course, there are certainly exceptions, but generally, it is more profitable to purchase a standard mold base and the related components.
Consistency- Since a good supplier will guarantee close tolerances, you can focus your attention on the cores and cavities, instead of more general machining. The profit is in the cores and cavities, so why spend time making something that can be purchased cheaper and made just as well, or perhaps better? Even if it costs a bit more, it still makes good business sense most of the time.
Delivery- a good supplier will deliver just what you want, on time. Plus, you don't have to pay for the scrapped parts he made while filling your order. This is especially true if you need more than one or two pieces.
Flexibility and innovation- companies are constantly searching for new and better ways to make components and improve their applications. Think about lifters, for example: there are numerous clever ways of making lifters, each one better than the rest! Take advantage of this collective inventiveness instead of relying on what worked 20 years ago.
Good communication- using standardized plastic injection mold components is often a selling point. Some companies have "favorite" suppliers for theirhot-runner system, for example. This also can make communication easier, since the features and sizes are catalogued and common.
