It’s because some of us used to be taught that when writing a letter (like with actual paper and pen or typewriter), you should always use a formal tone. Sometimes the sign off was longer than the actual letter:
*with the most profound respect, and with assurances of my highest consideration, your most obedient, most humble, and most devoted servant,

I have the honour to subscribe myself, Sir/Madam, your faithful and dutiful correspondent,

Respectfully and with sincere esteem, Mr Test_Tickles Esquire III*

Of course I’m being a little hyperbolic there, you get the idea. Once email appeared, people just weren’t going to do that shit anymore. But at the same time you don’t want to see me rude or angry, so you throw on a … to indicate all that other shit that we both know I’m supposed to write but I just don’t want to, but that’s cool because you don’t want to read it anyways.

Im in my late 30s and kind of do that. But only when segueing between loosely connected ideas, and usually when trying to convey that my comment isn’t the totality of the matter.

Ending a response with Thanks… would kind of imply theres a But to that thanks.

Thanks - I’m sure you’re right. They are in their 50s probably.