Opinions on this are a dime a dozen. You often see the reverse of it too, for example, you might have heard that "Python's standard library is where things go to die." You could just as easily call that a "terrible error." The fact that Python's standard library has an HTTP client in it, for example, doesn't stop everyone from using requests (and, consequently, urllib3) for all their HTTP client needs. So despite the fact the standard library provides a lot of the same
functionality as a third party dependency, folks are
still using the third party dependency.
I think the size of the standard library is just one of possibly many contributing factors that leads to a large number of dependencies. I think a part of it is culture, but another part of it is that the tooling _enables_ it. It's so incredibly easy to write some code, push it to crates.io and let everyone else use it. That's generally a good thing, but it winds up creating this spiral where there's almost no backpressure _against_ including a dependency in a project. This means there's very little standing in the way of letting the fullest expression of DRY run wild. There are some notable examples in the NPM ecosystem where it reaches ridiculous levels. But putting the extremes aside, there's a ton of grey area and it can be pretty difficult to convince someone to write a bit more code when something else might work off the shelf. (And I mean this in the most charitable way possible. I find myself in that situation.)
I do hope we can turn the Rust ecosystem around and stop regularly having dependency trees with hundreds of crates, but it's going to be a long and difficult road. For example, not everyone even agrees with my perspective that this is actually a bad thing.