Why are rear bike lights so useless?

Has anyone managed to find a quality rear bike light which will work for more than a few months? I have given up trying to find one and now just use cheap ones which can be easily replaced as they stop working (and they all stop). I also now cycle with multiple rear lights on the assumption one or two will stop working at some point. I have tried every major brand – Knog, Popeak, Cateye, Lezyne, etc. - and there seems to be a problem with them all. Many claim to be useable in bad weather, but then, after riding in the rain, they stop working. 

I am tired of returning lights to the online retailers (who are always very good about refunding or replacing - I suggest using Chain Reaction who have been great). Why can’t a waterproof bike light be produced which can be charged like my Garmin 500? The Garmin has worked for four years in all weather. There has never been a problem charging it through its little plug in. What is it about bike lights? The only rear lights I can absolutely depend on are those led strips which you can put around your arm. I now attach three of these to my bag and this reassures me that even when my rear light (inevitably) stops working, these will ensure drivers behind can see me. These, coupled with a hi-viz backpack cover, work very well. 

Again, why can’t bike light manufacturers make an all weather product with genuine durability? Can you imagine if the rear lights of cars were to stop working so easily on such a regular basis? People would be drive mad. If anyone can suggest a bike light they believe is as dependable as a Garmin, then send me the details and I will try it out. But I have to warn you of my growing cynicism. I am starting to believe bike lights come with a built in life span of a couple of months. Last time I bought a washing machine I was told it would last only a few years as “that’s the way they make them these days”. It seems there is something similar going on with bike lights. My Grandad used to cycle to work every day and had an old battery light which worked for over ten years. Why can’t such lights be made anymore? Bike lights don’t need to have mega lumens, they just need to make you visible to drivers. Give me something which works and works every time I need it to work.