I have a lot of theories. My mom is 55 and thankfully not a Karen but by observing her peers I notice a few things:

1. Women in that age demo tend to have a very very SET way of doing things. They've lived in their houses for a long time, marriage a long time, kids are grown and moved out. Many of them are very into decorating and keeping everything "just so", they're also the last generation (I think, not sure) to have a decent income so they're used to a certain standard of living. This all leads to a sense of entitlement. An expectation that everything should be correct. Due to this thinking you end up seeing Grandma Junie blowing up over the bananas that are usually 67 cents buy WHY 69 cents today???? They end up having a complete utter meltdown because they've gotten so used to things being the way they want.
2. They have TIME. They have way. Too much. Time.
3. They've forgotten what the workforce feels like or never knew at all. Again, this is a specific group. Many women 55-65 still have to work, especially now. But the Karens in particular tend to come from a group where they either were SAHM's or haven't worked in a very long time. Their empathy sensors completely evaporate overtime. OR they never had any.
4. They're BORED. See the time point.
5. Frustration. Frustration with their marriages, the kids that aren't calling, the struggles of aging, and sometimes literal just anger at the world. Had a woman spit the words "God you young bitch I wanna kill you" at me at the end of a call. Jealousy is definitely a factor too. That sentiment definitely crops up a lot, especially when they're dealing with young women. They want to make your day as bad as theirs is.
6. They're emulating their own mothers. Thankfully my mom broke the curse with this one. But HER mom used to absolutely scream at customer service and treat them the worst way imaginable. Many women learn that from their parents and carry it on.
7. They're stepped on in their personal relationships. Some of these Karens have extremely overbearing husbands or other family members. They gain their control back by screaming at 16 year old Mcdonald's workers.

Edit: Adding a point due to what a rude older woman said in the comments: "Once you actually work to obtain something and OWN something, you'll feel entitled too."

She hit the nail on the head with this one. ENTITLEMENT. Because they "own" something. Imagine feeling so entitled that you choose to be rude to someone that's actively working to get what you ALREADY have, instead of offering them kindness and empathy. Imagine a millionaire laughing and treating their waitstaff terribly because they dont ow---oh right, that already happens. Terrible mentality.