My belief is the controls are cheaper. I have installed a 5500w element in my home’s water heater after the element had failed. The control was rated for 4100 or 4500 I don’t remember, and within 6 months that control or thermostat went out. It was a high end model, so I replaced the controller and it only lasted 4 months. I called for a warranty (providing my license info) and they sent a new one. The new one also failed in 4 months, so I went through all the fun stuff again and the company said for $20 more I could upgrade to the commercial one. I told them they could send me that one on their dime; I said it was their warranty and was listed for the element they tried to convince me to pay the extra saying 4500watts is normal and I had to upgrade. Short version: they did send the commercial version after verification of my license (where they get home owners).
I had disassembled the first one that failed the contact sets were fairly small about the diameter of a pencil maybe slightly larger. The commercial version the contacts were replaceable and close to 1/2” square. That water heater was still running years later when I sold that house so I believe the reason most come with smaller heaters is they are on the edge of needing larger contacts and this costs more. My example is from the late 80’s but contact amperage would be a constant increase the load and a larger area in needed for long life.
The company tried to get out of the 3rd replacement by saying the instructions required a licensed electrician I faxed my card to them, that’s when I got the commercial replacement. It had a higher rating but I no longer remember what it was.
So I believe that they provide lower wattage elements for longer life. When the controller failed they wanted proof it was installed by a licensed electrician. I bet they do it on the first failure today.