The thing is, OP, that people only write about bad behaviour.
I am sure that I am not alone in having had the most wonderful MIL and PIL in the world. Unlike my DParents, they were kind, treated me as an adult, couldn’t do enough for my DC when they came along, came on holiday with us and were generally adored and supported by all of their DC and their partners until the day they died.
My DParents, on the other hand, were snobs, looked down on my DH and PILs, bad-mouthed my DH because of his profession and were reluctant to help (but needed help themselves, which, due to his work pattern, as they got older, my DH gladly ferried them around to hospital appointments, took them on days out with the DC while I was working and generally displayed all the traits he’d inherited from his DParents to become such a wonderful human being)
My PILs showed me how to be when my DC started bringing friends, then boy/girlfriends around - all the friends wanted to spend time at our house, especially when they needed support for whatever phase of growing up they were going through. Now I will be the same kind of MIL that mine was - and this is confirmed by my first affianced DC, so Thank you, MIL, for the fantastic example of in-law-hood that you gave me!