Plotly Software Engineer, Backend interview questions

It wasn't great I spoke to the recruiter and he was friendly. We spoke about my experience, education, the whole lot. I then got asked to create a project using NestJs, GraghQL, and some more technology which I did. It was reviewed and I moved to the next step where I got into a technical assessment. I passed everything there as well. I then got a rejection email and what it really said was that "The candidate has not worked on complicated tasks or has not proved himself working on complicated tasks" Now I don't have anything against rejections, but if you felt a candidate does not have enough experience, why did you put a candidate through 4 rounds of interviews and 4 weeks of preparation?