It's simply lithium-ion battery tech in general. They basically start to lose their ability to hold charge from the minute they are assembled at the factory. There are a million factors into how quickly they degrade, but in general, typically 2 years is the best I seem to get out of mine before I lose patience with them. Apple purposefully make it a pain in the dick to swap out the battery for monetary reasons. ("Come visit our Apple store and fork out $85!")
Meanwhile, I can order an actual OEM Samsung battery for $15 off Amazon (cheaper alternatives for non-OEM), then pull the old one out of my Galaxy phone and drop in the new in about 5 seconds flat.