Reddit is mostly text, with a few images for memes, very low in storage, processing power, or bandwidth, its easy to have a fediverse replacement.

Youtube is a video platform. Video take a lot of storage, encoding/decoding, bandwidth…

And video platforms are small amount of content creators, with 99 percent of the users being non-content-creating viewers. Once the content creators are on a monopoly platform (like youtube), there’s no incentive for them to leave. And fediverse wouldn’t have the same money to make.

In contrast, anyone can find interesting links to post to a Reddit-alternative, anyone can make memes.

TLDR: Reddit is community-focused. Youtube is creator-focused. Difference in hardware requirements. Theres no practical alternative for Youtube, especially not a fediverse one.