Why 4K HDR10 movies look way darker than their FullHD SDR 8-bit counterparts?

I have several 4K HDR10 footages and they all without exception look intolerably dark. I need to increase brightness to make it watchable. Look at these images before and after increasing luminance level: https://imgur.com/a/G1nFALB https://imgur.com/a/zMqGrlY And here is a 4K HDR10 vs FullHD SDR comparison of... show more I have several 4K HDR10 footages and they all without exception look intolerably dark. I need to increase brightness to make it watchable.

Look at these images before and after increasing luminance level:

https://imgur.com/a/G1nFALB
https://imgur.com/a/zMqGrlY

And here is a 4K HDR10 vs FullHD SDR comparison of the same frame:

https://imgur.com/a/GsDOCdJ

Why? Is the problem with the footage material? Is it the problem of the player? Is the problem of my 8-bit FullHD display? Or it is normal? Opinion?
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