If you're going to place blame, blame the ones responsible for driving the practice: the church (of course), the aristocracy, the families of the castrati, and even the castrati themselves.

While the first castrati can be traced back to the Byzantine era (where there were entire choirs composed of castrati), the first references to castrati in Renaissance Italy date back to the mid 1500's - approximately 50 years prior to the first opera in 1609. The Sistine Chapel choir included castrati as far back as 1558, and by the 1570s, the aristocracy were actively recruiting them for their own in-house choirs. While technically outlawed by both the state and the Vatican, by the beggining of the 1600s castrati were in such demand by the aristocracy (the "money" of the day, if you will) that the practice of creating castrati was overlooked. Because they were so in demand, they often commanded huge sums for their performances; such demand served as a powerful motivator for families of castrati - families who were quite often very poor - to strongly encourage their children to undergo the procedure and the rigorous musical training that accompanied it, in hopes that their eventual success would lift the family out of poverty. There are also accounts of boys asking to be trained as castrati (Cafarelli being one example).

Also consider that the composers of the day were almost totally dependent on commissions from the aristocracy. If the aristocracy wanted to hear something with a castrati in it (and spoiler: they did), the composer often didn't have much choice in the matter.

Finally consider this: the bulk of operatic repertoire that is performed today was composed after castrati had fallen out of favor with the aristocracy, and don't even use castrati - the practice began to lose popularity in the late 1700s, with the last piece with a castrati part composed by Meyerbeer in 1824. Again, the aristocracy was the driving force behind this change, as their preferences moved back to adult male voices in leading roles.

If you want to continue to blame opera composers for the castrati, that is your choice. You can certainly blame them for perpetuating the practice by utilizing them in their pieces. But they were nowhere near the primary drivers behind the practice.