Is christian altruism to blame for the failure to castrate the slaves?
Undoubtedly. The Moslems had no problem castrating their bucks.
It could also be simple Jewish greed. After all, having your farm tools make more farm tools saves you the trouble of buying more.
I have an admittedly shallow understanding of this subject, but I can try to answer this question.
African slavery in American didn't start to take off until the late 1600's. Until then, most farmwork was done by either subsistence famers who owned small plots of land, or by white indentured servants on larger plantations. It was almost a policy of Britain to use the American colonies as a dumping ground for undesirable/jobless British subjects, who could be pressed into farmwork in the colonies.
Americans had experimented with enslaving the native Indians, but this almost never turned out well for a variety of reasons, mostly that there just weren't enough of them and they tended to escape, for obvious reasons.
In 1676 a popular revolt against wealthy landowners in Virginia turned into a minor civil war known as
Bacon's Rebellion. While the revolt happened for several reasons, not all economical, what is true is that a large number of the rebels were poor whites, former indentured servants, and indentured servants themselves.
It's at this time that American landowners started actively looking for alternative labor sources. The Spanish controlled modern day Florida at this time, and already had substantial populations of African slaves. It was only a matter of time before the practice spread northward. The African slave was preferred over the white indentured servant for several reasons:
1. White servants, while bound by a contract of servitude, had legal rights. The African slave was regarded only as property.
2. White servants were legally required to be freed from their contract once it had been completed, African slaves had no path to freedom.
3. White servants might start getting uppity and start a revolt (see Bacon's rebellion). African slaves were much less likely to revolt, and even if they did, would find few people in the white population sympathetic to them.
This is at a time when there were vast amounts of untapped land on the American frontier. Crops like tobacco and cotton were bringing in massive amounts of wealth to the colonies, and further development of the land would naturally require a large labor force to tend to these farms. Far from castrating slaves, the incentives were in the exact opposite direction-the incentive was to purchase a number of slaves and then breed them in captivity in order for these future mega-farms to have enough labor.
The practice of castration among Ottoman slaves was for very different reasons than brute labor, these slaves would often become palace/harem guards. They were castrated so that they could not threaten the nobility in a sexual way, and they could not sire offspring that might compete for power among the nobility. Nothing like this system was present in the Americas.
https://iu.pressbooks.pub/theottomanharem/chapter/eunuchs-in-harems/
Also, as it may be obvious, castration surgery in 1670 was not the snip-snip as it is today. Castration surgery very often led to death. It would make no sense for American landowners to spend a significant sum of money to buy slaves, only for half of them to die in the castration process.
TLDR: Economic conditions of the time meant that there was no way that American slaveowners ever considered castrating their slaves. After all, they are a servile race, and there's absolutely no way they could ever achieve legal equality with whites, right?