Because it doesn’t really solve much. After every update of external libraries, do you go through all the diffs to see if there is malicious code? Of course you don’t. And even if you would, it’s not even always possible to spot it. So all locking packages does is postpone the problem to when you eventually update. As an added bonus, you’re now vulnerable to all the legitimate issues that get fixed in those updates you’re not installing regularly.

As LiPoly said, it doesn't really solve the problem. It's not useless, it does accomplish something, but not that. Locking dependencies isn't a security thing, it's a reproducible builds thing. You can accomplish that by just using a traditional static version of everything, but now you've got a maintenance headache as you're constantly needing to go in and update your dependency versions. You could instead use version ranging, but now you never actually know which version of a dependency any given build is going to end up using. Locking allows you to have the best of both worlds.

To understand how this works, lets take a look at a hypothetical. Lets say you have a code base, and a CICD setup. Additionally you're using a git-flow style release management where your release version is in master, your active development is in develop, and your feature work is done in feature branches. What you do is setup your version ranges to cover what the semantic versions of things say should be compatible (generally locked major version, and possibly locked minor depending on the library, but unlocked patch). In your CICD for CI builds of develop and feature branches you include a step that updates your lock file to the latest version of a library matching your version range. This insures that your develop and feature branches are always using the latest version of dependencies. For your master branch though, its CI job only builds, it never updates the lock file. This means when you merge a release to master your dependencies are effectively frozen. Every build from the master branch is guaranteed to use exactly the same versions of dependencies as when you merged it.