I think it's the rigor of the field. The general expectation is that a mechanical, or any other type of engineer, will bring to the table some minimum set of skills and knowledge that other engineers in the same field possess. This is not true in computing since so many people entered the field from so many vectors. Not to mention there's still a hell of a lot of snake oil being peddled because there's so much money in the field of computing.

For example: one can't be certain that someone with a Computer Science degree even knows how to program or has any other specific knowledge or skills. Or that someone supposedly with X years of experience with Y understands Y at all vs. just repeating tasks learned by rote. Or that someone who went to a Z bootcamp can actually do anything useful with/in Z. Maybe someday we'll get there, but it's still a long ways off.