Because an entire generation of aspiring programmers grew up blindly trusting code from others they never met in real life, and never actually bothered to read half the time. Given that all you need to do is infiltrate a transitive dependency, why wouldn't you ruthlessly exploit that attack vector if you're the type of piece of shit who is into that sort of thing?

The attacks will continue until they cease to work. They will only cease to work once we either A) start auditing one another as providers of dependencies; B) only uptake certain versions thereof after carefully reading them. Or C) make good enough stdlibs where we don't need a gajillion nested levels of dependency on dependency to get a project working sufficiently. Physical manufacturers actually do audit suppliers btw. That's a normal part of the QA loop that most software companies are completely unwilling to allocate manpower for.