Culture changes. I doubt most people do it out of disrespect, it's just how they are used to doing things.
It may reflect a lack of respect in the overall culture, I suppose.
I'm an old-fashioned guy, but I'm only thirty. I used to address older people as Mr. and Mrs., but I got too many wierd looks, so I quit doing that. They usually said, "Call me Bill," or what-have-you. I still do it at work, though. My "clients" are usually older folk.
Probably happened when business causal attire became everyday for jobs that used to require suits.
What is the point of the business suit? Especially in the summertime. Isn't the suit derived from European clothing and hence suited (pun intended) to European climates? I'm an American, and I sweat enough wearing a t-shirt, thank you very much. I know I'm a little wierd, but I just don't like wearing slacks. Jeans and BDU trousers are the only thing I'm satisfied with. Slacks fit too loosely, are too thin and the front pockets are useless. As soon as I sit down, everything falls out. Don't your four-inch fixed-blade knives, ink pens, change and cellphone all fall out as soon as you sit down? Mine do. Then the back pockets are so long, I can barely fish my wallet out.
Permanently jeaned, here,
fistful