Generally because you’re buying cheaper ones that aren’t built as well. Heat destroys LEDs and the cheap bulbs generally use fewer individual LEDs running at higher power to produce a given output in lumens. More expensive bulbs use more LEDs at lower power to achieve the same light output so that they’re not constantly being overdriven and last much longer.
I just buy a cheap jumbo pack from Amazon. They’re like 15 bucks and last for years which is good enough for me.