Perhaps it is from all the things they have been through, wisdom, losing their mind, or they just dont care anymore.
What ever it is, it makes them a great companion and sometimes less trouble.
My old cat Emma was very skittish when she was young. She freaked out about strangers visiting, new pets and pretty much anything different. As years went on and we went through tragedies, moves and other pets, she got so she wasn't scared of visitors, didn't care if I brought home a new pet and was fine with a move as long as there was a bed to sleep on.
Zoe That I have done a couple posts on, is still going strong as she lives at a blind cleaning shop. She sits up on her bed like she is the queen. She doesn't mind the noisy vans and ultrasonic machine. She doesn't attempt to run off when the garage door is open. Content to a short walk around the room and back to her blanket. Our dog, Cosmo a chihuahua is used to cats. When he comes in we head straight over to see Zoe and Cosmo licks her in the ear. She doesn't even mind. Probably happy to see someone her own size.
Yvette's old cat
A few times a year I baby sit Yvette's pets. She has had this old lady since before I first knew her many years ago. I cant remember her name, when I do I will fix it here. Anyways she loves every bit of attention she can get. She barely weights a pound. Recently I helped Yvette move to a different area. I put all the critters in my van. When we got to the new house, we took all the cats to one room so they could come out of the carriers. This old lady popped right out and check out the new view from the window. The other 5 younger ones continued to cower in their carriers for a while.
If you no and easy going senior kitty, please tell us about them in comments.