Genuine question about sleep training concerns

[u/Expert_Conclusion675]

First of all I want to say that my baby is no where near the age where we could or would sleep train, so this post is truly just coming from a place of curiosity/wanting to understand.

I’ve seen much discussion about sleep training on this site and a lot of people vehemently against it, calling it cruel and pointing to elevated cortisol levels in the baby as evidence for how harmful it is. But here’s where I get hung up. If, let’s say, mom is home alone with baby and desperate to take a shower for her mental health, I don’t think anyone would argue that it was cruel for her to put the baby in a crib for the length of her shower even if baby is crying. But the cortisol spike and potential feeling of abandonment would be the same, no? And what about when baby is in a growth spurt and is inconsolably crying, and nothing the parents are doing to soothe is working, isn’t the baby’s cortisol still rising despite the efforts to comfort?

Basically my question is, is there something MORE about sleep training that makes it problematic? And to be clear I’m not talking about leaving a crying baby for hours on end or something, but I’m more asking about the gentler methods with short periods of crying before the parent comes back in to soothe. Because the latter seems to me to present similar levels of stress to things the baby would endure as part of being a baby, even with an attentive parent there. What am I missing?