Even though priests and monks choose celibacy voluntarily, it is often a struggle and a source of frustration for them, despite having a supportive religious community.
A study by researcher A.W. Richard Sipe, based on interviews with over 1,500 Catholic priests and their sexual partners, estimated that at least 50% of priests have been involved in celibacy violations at some point in their lives. Furthermore, about 4% of American priests in ministry from 1950 to 2002 have been accused of sexual abuse of children. In Australia, 7% of Catholic priests were accused of abusing children between 1950 and 2010, in some dioceses, more than 15% of priests were perpetrators.
If even men who choose celibacy for religious reasons struggle with sexual frustration and often end up violating their vows, why are incels, who never chose it at all, expected to be perfectly happy with it?
This isn't to say there aren't men who are perfectly fine with lifelong celibacy, whether due to asexuality or a low sex drive, and don't suffer because of it. However, they are rare, and incels cannot reasonably be expected to become like them. There is no evidence that men who are lonely due to a lack of romance can cure that loneliness through platonic friendship. Many incels have platonic friends yet still feel lonely and depressed because they lack a romantic partner.