What is bad about human shampoos that makes it so you can't use it for dogs?

From what I understand from a groomer friend I think it’s the ph levels.  That’s what I took from the info that was given to me at least haha. 

I believe it is the Ph levels however in a pinch baby shampoo can be used or even just standard human shampoo. Its not great as a long term wash product. I remember when I did my grooming course that we did use diluted baby shampoo to wash the face in case we accidentally got in in the dogs eyes as the baby shampoo wouldnt sting as much as the human or even the dog shampoo would. Now I am not a groomer at all thats just the information that was given to me.

I think it strips protective oils and leaves them susceptible to rashes. I’ve used just aloe Vera conditioner on occasion without calamity.