Because there is a whole industry around hyping PKM systems and note taking systems with pretty weak evidence to support their actual utility. Calling them essential creates the impression that those not using them are deficient.
Yet, given that people are both massively productive with and without these systems, it really calls into question the effusive marketing and community promotion of these tools.
I use obsidian everyday, but I recognize that I could do just as well with some other tool. It's preferential, not essential.
Exaggeration is not necessary for communication of value.
