It seems that most dependencies of cargo-watch are pulled from three direct requirements: clap, cargo_metadata and watchexec. Clap would pull lots of CLI things that would be naturally platform-dependent, while cargo_metadata will surely pull most serde stuffs. Watchexec does have a room for improvement though, because it depends on command-group (maintained in the same org) which unconditionally requires Tokio! Who would have expected that? Once watchexec got improved on that aspect however, I think these requirements are indeed necessary for the project's goal and any further dependency removal will probably come with some downsides.
A bigger problem here is that you can't easily fix other crates' excessive dependencies. Watchexec can be surely improved, but what if other crates are stuck at the older version of watchexec? There are some cases where you can just tweak Cargo.lock to get things aligned, but generally you can't do that. You have to live with excessive and/or duplicate dependencies (not a huge problem by itself, so it's default for most people) or work around with `[patch]` sections. (Cargo is actually in a better shape given that the second option is even possible at all!) In my opinion there should be some easy way to define a "stand-in" for given version of crate, so that such dependency issues can be more systematically worked around. But any such solution would be a huge research problem for any existing package manager.