Your water heaters should last longer, which you know.
If you still have access to the old water heater, take a look at the anode rod. This may take an impact wrench. If it is all eaten away, I would be thinking of using a powered anode. I put one in. Not cheap. I am not saying that this will cure your water heater life, but it does act like an always-new anode. You can also get the anode rod replaced in a couple of years. If when you do that, the old one is eaten away, do it sooner next time, or consider a powered anode.
Maybe you have just had a spate of bad luck with water heaters. I hope you are getting some warranty coverage. I am not a pro.