For one, storing multiple versions of the same film takes up a lot of storage, which is more expensive than a cheap 40€ gpu for transcoding. And I definitely wanna keep the highest quality I can. Besides transcoding on the fly is more flexible, ensuring the best possible quality at any time, instead of having to pick between the good and the shit version.
And secondly, usually I only need transcoding when I don’t watch on my home setup (or when some friends watch on my server). My upload isn’t as high as some of my film’s bitrates and some clients do not support h.265 or HDR thus needing transcoding and/or tonemapping.