1. On Windows and Linux, devices have to handle a huge number of hardware configurations. Not everything can go into the same "deep sleep" mode with the same instructions, so developers implement different "idle" states that may or may not discharge the battery quickly. Too many sales efforts focus on making a laptop 'boot' from idle in seconds, and too few focus on making it usable after being off for days. Look up "S3 Deep Sleep", "Modern Standby", and the bevy of options described at https://learn.microsoft.com/en-us/windows-hardware/drivers/k... to learn more. Apple only supports a small list of hardware platforms, and can make deep sleep work and resume quickly and reliably on just that list. Even the Kindle Fire is not totally vertically integrated between the MediaTek processor and Android OS and Amazon UI, there's bound to have been a lot of finger-pointing and little leadership pushing for microamp-level sleep capabilities.
2. Android/Windows/Linux allow users to install applications that do things that consume battery in the background - for example, my Tile bluetooth locators and Garmin smartwatch work great with my Android phones. They wake up a sleeping phone and force it to use a little power to grab a location data point or send my watch an updated weather forecast. Apple doesn't let third-party developers do the same.
I'd bet that if you pushed and held the power button on your Kindle tablet on Sunday night for 3 seconds and chose a full "Shut Down", and then held the button again Friday for 2 seconds, it would have the same battery level as it had when you turned it off. It's just because it's been checking for system/app updates and otherwise running code while 'sleeping' unused that the batteries are low, in spite of the fact that no humans were using it.