Slack interview questions

I had a phone interview with the recruiter. the interview was technical, and he asked about my resume. he folllowed up and also asked about how the internet works, and defining technical terms like udp/tcp and giving examples of when to use either. I also got asked about UI related terminology and user experience problems. The interview itself was fair, but I have to say, the quality of the voice and background noise was terrible - it sounded like a call center in there.

A HR person from slack emailed me to set an interview for Friday at 5:00 pm. They never called at the time. I emailed them and on Monday they get back to me saying "sorry I was sick, we can reschedule again". No big deal so far. Finally we get to talk on Tuesday. She assigned me 2 coding problems (specific to Slack) which I had to pick one and had a week to do. I finished it and sent it on Friday. Since I did not get verification that they got my code, I sent an email on Monday. The HR person emailed me back that she missed my email, and she will make sure to see if the project manager got it. A day after which was on Tuesday( exactly one week after they assigned me a coding project), they emailed me that they can not offer me the internship. I emailed them to ask why since the code was working perfectly, of course I never heard back from them. My concerns: 1. Why do they asked me to write a script to test some functionality of their product and not giving me a generic coding questions like other legit companies? 2. Is the answer to the first question is "because they want to get their job done for FREE?" 3. Why did they never give me reasonable answer WHY they did not hire me if I did whatever they asked for? 4. They gave me 1 week to do the code, but since I finished in 3 days, they took less than 1 day to decide they dont want to offer me. It means that they did not want to hire me at the first place. REFER to number 2.